pkgconfig: Ensure pkgconfig RDEPENDS are tracked

Currently, pkgconfig dependencies get added to -dev packages which install .pc files however nothing in the system makes bitbake aware of these dependencies so images can fail with pkgconfig being missing.

This change explictly adds in the pkgconfig RDEPENDS to the -dev packages and hence makes bitbake aware of the dependency.

---

index a12cd64..53e793a 100644
@@ -1 +1,6 @@
 DEPENDS_prepend = "pkgconfig-native "
+
+PKGCONFIGRDEP = "pkgconfig"
+PKGCONFIGRDEP_virtclass-native = ""
+
+RDEPENDS_${PN}-dev += "${PKGCONFIGRDEP}"