polar_chain_test: fix compilation issue when AVX2 isn't used

this fixes compilation on aarch64:

FAILED: lib/src/phy/fec/polar/test/CMakeFiles/polar_chain_test.dir/polar_chain_test.c.o
/usr/bin/gcc -DENABLE_TIMEPROF -DENABLE_TTCN3 -DHAVE_5GNR -DHAVE_MBEDTLS -I/usr/include/SoapySDR -Ilib/include -I../lib/include -Wno-unused-but-set-variable -Wall -Wno-comment -Wno-write-strings -Wno-unused-result -Wformat -Wmissing-field-initializers -Wtype-limits -std=c99 -fno-strict-aliasing -D_GNU_SOURCE -O3 -fno-trapping-math -fno-math-errno -DBUILD_TYPE_RELEASE -DIS_ARM -DHAVE_NEON -DHAVE_NEONv8 -fvisibility=hidden -fdiagnostics-color=always -Werror --param large-function-growth=1600 -O3 -DNDEBUG -fPIE -MD -MT lib/src/phy/fec/polar/test/CMakeFiles/polar_chain_test.dir/polar_chain_test.c.o -MF lib/src/phy/fec/polar/test/CMakeFiles/polar_chain_test.dir/polar_chain_test.c.o.d -o lib/src/phy/fec/polar/test/CMakeFiles/polar_chain_test.dir/polar_chain_test.c.o   -c ../lib/src/phy/fec/polar/test/polar_chain_test.c
../lib/src/phy/fec/polar/test/polar_chain_test.c: In function ‘main’:
../lib/src/phy/fec/polar/test/polar_chain_test.c:214:11: error: unused variable ‘gain_c_avx2’ [-Werror=unused-variable]
  214 |   float   gain_c_avx2 = NAN;
      |           ^~~~~~~~~~~
../lib/src/phy/fec/polar/test/polar_chain_test.c:191:7: error: unused variable ‘errors_symb_c_avx2’ [-Werror=unused-variable]
  191 |   int errors_symb_c_avx2 = 0;
      |       ^~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

lib/src/phy/fec/polar/test/polar_chain_test.c

@@ -188,7 +188,9 @@ int main(int argc, char** argv)
   int errors_symb        = 0;
   int errors_symb_s      = 0;
   int errors_symb_c      = 0;
+#ifdef LV_HAVE_AVX2
   int errors_symb_c_avx2 = 0;
+#endif
 
   int n_error_words[SNR_POINTS + 1];
   int n_error_words_s[SNR_POINTS + 1];
@@ -211,7 +213,9 @@ int main(int argc, char** argv)
   int8_t  inf8        = (1U << 7U) - 1;
   float   gain_s      = NAN;
   float   gain_c      = NAN;
+#ifdef LV_HAVE_AVX2
   float   gain_c_avx2 = NAN;
+#endif
 
   srslte_polar_code_t    code;
   srslte_polar_encoder_t enc;