archivers/pxz: fix build on powerpc

cc -o pxz -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -fopenmp -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE pxz.c -fstack-protector-strong -llzma -DPXZ_BUILD_DATE=\"date +%Y%m%d\" -DPXZ_VERSION=\"4.999.9beta\"
pxz.c:48:10: fatal error: 'omp.h' file not found