pkgconfig: Add private zlib dependency for libgsttag

Fixes issues with static application builds (which might not know
they need to link with zlib when using a libgsttag that was compiled
with it).

https://bugzilla.gnome.org/show_bug.cgi?id=793989

pkgconfig/gstreamer-tag-uninstalled.pc.in

@@ -12,5 +12,6 @@ Description: Tag base classes and helper functions, uninstalled
 Version: @VERSION@
 Requires: gstreamer-@GST_API_VERSION@
 Libs: -L${libdir} -lgsttag-@GST_API_VERSION@
+Libs.Private: @ZLIB_LIBS@
 Cflags: -I@abs_top_srcdir@/gst-libs -I@abs_top_builddir@/gst-libs
 

pkgconfig/gstreamer-tag.pc.in

@@ -12,5 +12,6 @@ Description: Tag base classes and helper functions
 Requires: gstreamer-@GST_API_VERSION@
 Version: @VERSION@
 Libs: -L${libdir} -lgsttag-@GST_API_VERSION@
+Libs.private: @ZLIB_LIBS@
 Cflags: -I${includedir}