CMake: Fix build on Solaris the stack_protector feature enabled

Solaris requires the ssp library to be linked when
-fstack-protector-strong is passed to the compiler.

Pick-to: 6.8
Fixes: QTBUG-129085
Change-Id: I55396fd9dc102ffd98bef54fc3dfc1aadf18a404
Reviewed-by:  Alexey Edelev <alexey.edelev@qt.io>

cmake/QtInternalTargets.cmake

@@ -333,6 +333,9 @@ endif()
 
 if(QT_FEATURE_stack_protector)
     target_compile_options(PlatformCommonInternal INTERFACE -fstack-protector-strong)
+    if(CMAKE_SYSTEM_NAME STREQUAL "SunOS")
+        target_link_libraries(PlatformCommonInternal INTERFACE ssp)
+    endif()
 endif()
 
 if(QT_FEATURE_stack_clash_protection)