Add missing Cflags entry to pkg-config file

It should actually be -I${includedir}/lxappearance, but including
<lxappearance/lxappearance.h> has apparently become common in dependent
projects.

Author: ib

data/lxappearance.pc.in

@@ -8,3 +8,4 @@ Name: lxappearance
 Description: Tool used to customize look and feel of gtk+ applications.
 Requires: @gtk_modules@
 Version: @VERSION@
+Cflags: -I${includedir}