build: Add @LZMA_LIBS@ to libxml’s pkg-config files

If libxml2 is built and linked against liblzma, the latter needs to appear in
libxml2’s Libs.private pkg-config field, otherwise static linking against
libxml2 will fail due to unresolved liblzma symbols.

https://bugzilla.gnome.org/show_bug.cgi?id=711026
diff --git a/libxml-2.0.pc.in b/libxml-2.0.pc.in
index f5f5f03..2653a7c 100644
--- a/libxml-2.0.pc.in
+++ b/libxml-2.0.pc.in
@@ -9,5 +9,5 @@
 Description: libXML library version2.
 Requires:
 Libs: -L${libdir} -lxml2
-Libs.private: @ICU_LIBS@ @THREAD_LIBS@ @Z_LIBS@ @ICONV_LIBS@ @M_LIBS@ @WIN32_EXTRA_LIBADD@ @LIBS@
+Libs.private: @ICU_LIBS@ @THREAD_LIBS@ @Z_LIBS@ @LZMA_LIBS@ @ICONV_LIBS@ @M_LIBS@ @WIN32_EXTRA_LIBADD@ @LIBS@
 Cflags: @XML_INCLUDEDIR@ @XML_CFLAGS@