Fix GMMLIB_API_PATCH_VERSION setting when not defined as external

Fixes variable value dereference in cmake. Previously
PATCH_VERSION was used literally instead of variable value.

Change-Id: Ied1b2b86c94192c5493d89c8102ff765da2fae52
Signed-off-by: Artur Harasimiuk <artur.harasimiuk@intel.com>
diff --git a/Source/GmmLib/CMakeLists.txt b/Source/GmmLib/CMakeLists.txt
index 97b84b6..4eadbef 100644
--- a/Source/GmmLib/CMakeLists.txt
+++ b/Source/GmmLib/CMakeLists.txt
@@ -40,7 +40,7 @@
 endif()
 
 if(NOT DEFINED GMMLIB_API_PATCH_VERSION)
-    set(GMMLIB_API_PATCH_VERSION PATCH_VERSION)
+    set(GMMLIB_API_PATCH_VERSION ${PATCH_VERSION})
 endif()
 
 message(STATUS "API version: ${GMMLIB_API_MAJOR_VERSION}.${GMMLIB_API_MINOR_VERSION}.${GMMLIB_API_PATCH_VERSION}")