Add prefix_front_back. to get prefix_front_ and prefix_back_

[liujiayi771]

Encountered a compilation error while compiling re2. The compilation succeeds by adding the prefix prefix_front_back. to prefix_front_ and prefix_back_.

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[github-actions]

RE2 uses Gerrit instead of GitHub pull requests.
See the Contribute wiki page.

[liujiayi771]

We encountered this error while compiling the re2 code in DuckDB's third_party directory, and we hope to have it fixed upstream.

FAILED: third_party/re2/CMakeFiles/duckdb_re2.dir/re2/prog.cc.o
/usr/bin/ccache /usr/lib64/ccache/c++ -DEXT_VERSION_JEMALLOC=\"c3903e44e0\" -DEXT_VERSION_PARQUET=\"c3903e44e0\" -DRE2_ON_VALGRIND -I/tmp/duckdb/src/include -I/tmp/duckdb/third_party/fsst -I/tmp/duckdb/third_party/fmt/include -I/tmp/duckdb/third_party/hyperloglog -I/tmp/duckdb/third_party/fastpforlib -I/tmp/duckdb/third_party/skiplist -I/tmp/duckdb/third_party/fast_float -I/tmp/duckdb/third_party/re2 -I/tmp/duckdb/third_party/miniz -I/tmp/duckdb/third_party/utf8proc/include -I/tmp/duckdb/third_party/concurrentqueue -I/tmp/duckdb/third_party/pcg -I/tmp/duckdb/third_party/tdigest -I/tmp/duckdb/third_party/mbedtls/include -I/tmp/duckdb/third_party/jaro_winkler -I/tmp/duckdb/third_party/yyjson/include -mavx2 -mfma -mavx -mf16c -mlzcnt -std=c++17 -mbmi2 -O3 -DNDEBUG -O3 -DNDEBUG   -std=c++17 -fPIC -fvisibility=hidden -std=c++11 -w -MD -MT third_party/re2/CMakeFiles/duckdb_re2.dir/re2/prog.cc.o -MF third_party/re2/CMakeFiles/duckdb_re2.dir/re2/prog.cc.o.d -o third_party/re2/CMakeFiles/duckdb_re2.dir/re2/prog.cc.o -c /tmp/duckdb/third_party/re2/re2/prog.cc
/tmp/duckdb/third_party/re2/re2/prog.cc: In member function 'const void* duckdb_re2::Prog::PrefixAccel_FrontAndBack(const void*, size_t)':
/tmp/duckdb/third_party/re2/re2/prog.cc:1146:45: error: 'prefix_front_' was not declared in this scope
 1146 |     const __m256i f_set1 = _mm256_set1_epi8(prefix_front_);
      |                                             ^~~~~~~~~~~~~
/tmp/duckdb/third_party/re2/re2/prog.cc:1147:45: error: 'prefix_back_' was not declared in this scope; did you mean 'prefix_size_'?
 1147 |     const __m256i b_set1 = _mm256_set1_epi8(prefix_back_);
      |                                             ^~~~~~~~~~~~
      |                                             prefix_size_