bugfix: mircommon-internal-dev was erroneously including some libs (#…

…3366)

My local version of `pkgconfig` doesn't complain about this, but it is
most definitely an error!

src/common/mircommon-internal.pc.in

@@ -2,7 +2,7 @@ prefix=@CMAKE_INSTALL_PREFIX@
 includedir=@PKGCONFIG_INCLUDEDIR@/mircommon-internal
 
 Name: mircommon-internal
-Description: Mir server internal headers
+Description: Mir common internal headers
 Version: @MIR_VERSION@
-Libs: -L${libdir} -lmircommon
+Requires: mircommon mircore
 Cflags: -I${includedir}