RSS: Fix cmake path error

Fix issue where path inside generator expression was not absolute.

Change-Id: I1155014a779b21eb610c6756f491bd88acd6acf2
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/platform/ext/target/arm/rss/common/fip_parser/CMakeLists.txt b/platform/ext/target/arm/rss/common/fip_parser/CMakeLists.txt
index 60c26ff..a858d90 100644
--- a/platform/ext/target/arm/rss/common/fip_parser/CMakeLists.txt
+++ b/platform/ext/target/arm/rss/common/fip_parser/CMakeLists.txt
@@ -11,7 +11,7 @@
     PRIVATE
         fip_parser.c
         host_flash_atu.c
-        $<$<BOOL:${RSS_GPT_SUPPORT}>:gpt.c>
+        $<$<BOOL:${RSS_GPT_SUPPORT}>:${CMAKE_CURRENT_SOURCE_DIR}/gpt.c>
 )
 
 target_include_directories(platform_bl2
@@ -25,7 +25,7 @@
     PRIVATE
         fip_parser.c
         host_flash_atu.c
-        $<$<BOOL:${RSS_GPT_SUPPORT}>:gpt.c>
+        $<$<BOOL:${RSS_GPT_SUPPORT}>:${CMAKE_CURRENT_SOURCE_DIR}/gpt.c>
 )
 
 target_include_directories(platform_bl1_1_interface