Build: Fix TEST_PSA_API HEAP_MEM setting

Fix setting syntax to properly disable heap support.

Change-Id: I78cddf05408c7e5960d7ddc42fef3842541fe108
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/app/CMakeLists.txt b/app/CMakeLists.txt
index b7e5d73..e89bf66 100644
--- a/app/CMakeLists.txt
+++ b/app/CMakeLists.txt
@@ -42,7 +42,7 @@
     endif()
 
     if(NOT SP_HEAP_MEM_SUPP)
-        set(SP_HEAP_MEM_SUPP=0)
+        set(SP_HEAP_MEM_SUPP 0)
     endif()
     if(NOT PLATFORM_PSA_ISOLATION_LEVEL)
         set(PLATFORM_PSA_ISOLATION_LEVEL ${TFM_ISOLATION_LEVEL})