liblzma: add -ldl to Libs.private to support static linking again

liblzma: add -ldl to Libs.private to support static linking again

Static linking using the flags suggested by pkg-config fails:

	$ gcc -static -o xz_pipe_comp xz_pipe_comp.c \
		$(pkg-config --static --cflags --libs liblzma)
	/usr/lib/i386-linux-gnu/liblzma.a(liblzma_la-common.o): In function `lzma_strm_init':
	(.text+0x32a): undefined reference to `dlopen'

Add -ldl to get that working again.

After this patch, gcc still warns:

	Using 'dlopen' in statically linked applications requires at
	runtime the shared libraries from the glibc version used for
	linking

This particular use of 'dlopen' is not harmful because it is only
meant to check if liblzma.so.2 is already (dynamically) loaded.  It is
also not useful in statically linked applications, but fixing that can
wait for another patch.

Reported-by: Lasse Collin <lasse.collin@tukaani.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>