weston.pc: Added libexecdir and pkglibexecdir variables

These can be used by external clients to check the installation path

Signed-off-by: Eduardo Lima (Etrunko) <eduardo.lima@intel.com>
diff --git a/src/weston.pc.in b/src/weston.pc.in
index 537cc89..828cb1f 100644
--- a/src/weston.pc.in
+++ b/src/weston.pc.in
@@ -2,6 +2,8 @@
 exec_prefix=@exec_prefix@
 libdir=@libdir@
 includedir=@includedir@
+libexecdir=@libexecdir@
+pkglibexecdir=${libexecdir}/@PACKAGE@
 
 Name: Weston Plugin API
 Description: Header files for Weston plugin development