perl: Pass @CFLAGS@ through extra_linker_flags

The documentation for Module::Build is completely opaque on how you're
supposed to pass @CFLAGS@ correctly.  However I noticed that we were
not passing -g through when generating Guestfs.so:

gcc -lpthread -shared -Wl,-z,relro -Wl,--as-needed -Wl,-z,pack-relative-relocs -Wl,-z,now '-specs=/usr/lib/rpm/redhat/redhat-hardened-ld' '-specs=/usr/lib/rpm/redhat/redhat-annobin-cc1' '-Wl,--build-id=sha1' -L/usr/local/lib -fstack-protector-strong -lperl -o blib/arch/auto/Sys/Guestfs/Guestfs.so lib/Sys/Guestfs.o -L../lib/.libs -lguestfs

This debuginfo was not always being generated correctly.  (For some
reason it still manages to be generated in Fedora and RHEL 9, but not
in RHEL 10, this part is still unclear to me.)

Anyway it seems we ought to pass at least -g to the linker, and we
might as well pass the full set of @CFLAGS@, hence this change.

perl/Build.PL.in

@@ -65,6 +65,8 @@ my $build = Module::Build->new (
         '@top_srcdir@/include',
     ],
     extra_linker_flags => [
+        '-DGUESTFS_PRIVATE=1',
+        split (' ', '@CFLAGS@'),
         '-L@top_builddir@/lib/.libs',
         '-lguestfs',
     ],