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

For commit b0dcdcb ("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: b0dcdcb ("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>

Author: pchaigno

tools/testing/selftests/bpf/Makefile

@@ -409,7 +409,7 @@ $(RESOLVE_BTFIDS): $(HOST_BPFOBJ) | $(HOST_BUILD_DIR)/resolve_btfids	\
 		CC="$(HOSTCC)" LD="$(HOSTLD)" AR="$(HOSTAR)" \
 		LIBBPF_INCLUDE=$(HOST_INCLUDE_DIR) \
 		EXTRA_LDFLAGS='$(SAN_LDFLAGS) $(EXTRA_LDFLAGS)' \
-		HOSTPKG_CONFIG=$(PKG_CONFIG) \
+		HOSTPKG_CONFIG='$(PKG_CONFIG)' \
 		OUTPUT=$(HOST_BUILD_DIR)/resolve_btfids/ BPFOBJ=$(HOST_BPFOBJ)
 
 # Get Clang's default includes on this system, as opposed to those seen by