pkgconfig: inherit the pkgconfig class

pkgconfig: inherit the pkgconfig class

Building target pkgconfig needs pkgconfig-native because pkgconfig uses
pkgconfig to find libglib.  Inherit the pkgconfig class so that the
pkgconfig recipe does not need to know the details of how pkgconfig is
integrated.

Native builds use an internal copy of glib to avoid the circular
dependency, and the self-dependency on pkgconfig-native is automatically
pruned.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>