selftests/bpf: Fix pkg-config call on static builds

For commit b0dcdcb9ae75 ("resolve_btfids: Fix linker flags detection"),
I suggested setting HOSTPKG_CONFIG to $PKG_CONFIG when compiling
resolve_btfids, but I forgot the quotes around that variable.

As a result, when running vmtest.sh with static linking, it fails as
follows:

$ LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh
[...]
make: unrecognized option '--static'
Usage: make [options] [target] ...
[...]

This worked when I tested it because HOSTPKG_CONFIG didn't have a
default value in the resolve_btfids Makefile, but once it does, the
quotes aren't preserved and it fails on the next make call.

Fixes: b0dcdcb9ae75 ("resolve_btfids: Fix linker flags detection")
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/abADBwn_ykblpABE@mail.gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>