acl: fix compiler flags with meson and AVX2 runtime

acl: fix compiler flags with meson and AVX2 runtime

When compiling the ACL library on a system without AVX2 support,
the flags used to compile the AVX2-specific code for later run-time
use were not based on the regular cflags for the rest of the library.
This can cause errors due to symbols being missed/undefined
due to incorrect flags. For example,
when testing compilation on Alpine linux, we got:
	error: unknown type name 'cpu_set_t'
due to _GNU_SOURCE not being defined in the cflags.

This issue can be fixed by appending "-mavx2" to
the cflags rather than replacing them with it.

Fixes: 5b9656b157d3 ("lib: build with meson")
Cc: stable@dpdk.org
Signed-off-by: Andrius Sirvys <andrius.sirvys@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>