Remove unnecessary system include path from pkg-config files.

Originally committed as revision 11247 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2081,7 +2081,7 @@ Version: $version
 Requires: $requires
 Conflicts:
 Libs: -L\${libdir} $libs
-Cflags: -I\${includedir} -I\${includedir}/$include
+Cflags: -I\${includedir}/$include
 EOF
 }