libkmod: Fix unaligned access

From Jan Engelhardt:

	Program received signal SIGBUS, Bus error.
	[Switching to process 11100]
	0x00035278 in kmod_module_signature_info (file=0x4eeb8, sig_info=0xffffc254)
	    at libkmod/libkmod-signature.c:124
	124             sig_len = be32toh(modsig->sig_len);
	(gdb) p modsig
	$1 = (const struct module_signature *) 0xf7dfe143

modsig->sig_len can be unaligned if modsig is unaligned, so the padding
in the struct has no effect since we are mapping it to the mem buffer.
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.