Build: Skip building platform_ns when NS app is not selected

Set EXCLUDE_FROM_ALL property in platform_ns target to skip
building platform_ns target when NS app is disabled.

Change-Id: I2e9c5122ec3253d510806d5cdf3d18ee3e4b1985
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/platform/CMakeLists.txt b/platform/CMakeLists.txt
index f7fd6ef..6cd9bef 100755
--- a/platform/CMakeLists.txt
+++ b/platform/CMakeLists.txt
@@ -14,7 +14,7 @@
 # under the multicore topology, NS is declared in the same place as the PSA
 # interface so that it picks up the compiler definitions for the NS CPU
 if(NOT TFM_MULTI_CORE_TOPOLOGY)
-    add_library(platform_ns STATIC)
+    add_library(platform_ns STATIC EXCLUDE_FROM_ALL)
 endif()
 
 if (BL2)