BUILD: fix ssl_sample.c when building against BoringSSL

BUILD: fix ssl_sample.c when building against BoringSSL

BoringSSL does not have X509_get_X509_PUBKEY
let our emulation level define that for BoringSSL as well

Build log:

src/ssl_sample.o: In function `smp_fetch_ssl_x_key_alg':
/home/travis/build/haproxy/haproxy/src/ssl_sample.c:592: undefined reference to `X509_get_X509_PUBKEY'
clang-7: error: linker command failed with exit code 1 (use -v to see invocation)
Makefile:860: recipe for target 'haproxy' failed
make: *** [haproxy] Error 1

travis-ci: https://travis-ci.com/github/haproxy/haproxy/jobs/351670996