Fix include path in xrdp-uninstalled.pc.in

${pcfiledir} refers to the directory of the package-config file. For an
xrdp build in the source tree, the correct include location would be
${pcfiledir}/../common, not ${pcfiledir}/common

For xrdp build outside the source tree, there is no way to find the
source tree without help from the configure script.

While the '${pc_top_builddir}/${pcfiledir}' pattern is still widespread,
many packages (e.g. gstreamer) are switching to @abs_top_srcdir@ and
@abs_top_builddir@, which are always absolute and have knowledge about
the source tree location from the configure script.

Use @abs_top_srcdir@ for the headers. @abs_top_builddir@ is not needed,
as xrdp produces no externally consumable libraries or generated
headers.

pkgconfig/xrdp-uninstalled.pc.in

@@ -1,4 +1,5 @@
-includedir=${pc_top_builddir}/${pcfiledir}/common
+abs_top_srcdir=@abs_top_srcdir@
+includedir=${abs_top_srcdir}/common
 
 Name: xrdp
 Description: An open source Remote Desktop Protocol (RDP) server