build-sys: try harder to shut up compiler wrt strict-aliasing

build-sys: try harder to shut up compiler wrt strict-aliasing

With -Wstrict-aliasing=2 we get the following warning:

	libkmod/libkmod-signature.c:124:20: note: in expansion of macro 'get_unaligned'
	  sig_len = be32toh(get_unaligned(&modsig->sig_len));

However there's nothing wrong with it. modsig->sig_len is uint32_t and
get_unaligned in this case returns uint32_t. There's notstrict aliasing
violation.