pong-clock: Add missing 'inherit pkgconfig'

The usage of pkg-config is sneakily hidden in do_compile()

(From OE-Core rev: b55186f93163e120c73bacb7400b94c21cfaa675)

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-graphics/pong-clock/pong-clock_1.0.bb

@@ -2,7 +2,7 @@ SUMMARY = "A clock combined with a game of pong"
 LICENSE = "GPLv2+"
 DEPENDS = "virtual/libx11 xdmcp xau"
 
-inherit distro_features_check
+inherit distro_features_check pkgconfig
 # depends on virtual/libx11
 REQUIRED_DISTRO_FEATURES = "x11"