capstone.pc.in: allow absolute CMAKE_INSTALL_*DIR

This patch fixes Capstone 5 build on NixOS.

NixOS's build infrastructure sets CMAKE_INSTALL_{LIB,INCLUDE}DIR to
absolute paths. If you append it to ${prefix}, you get the wrong path.
NixOS automatically detects it and links this issue:
NixOS/nixpkgs#144170

capstone.pc.in

@@ -1,7 +1,7 @@
 prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}
-libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
+includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
 
 Name: capstone
 Description: Capstone disassembly engine