Tests: Simplify VSGNUFortran Oracle-specific link lines

On an Oracle 12.4 build the c_using_fortran executable cannot find the
"fsu" library at runtime.  Since this is an implementation detail of the
"hello" library, link that library to it privately so that "-lfsu" does
not propagate to the executables consuming it.

Tests/VSGNUFortran/subdir/fortran/CMakeLists.txt

@@ -35,12 +35,12 @@ add_library(hello SHARED hello.f)
 add_library(world SHARED world.f)
 target_link_libraries(hello world)
 if(CMAKE_Fortran_COMPILER_ID MATCHES SunPro)
-  target_link_libraries(hello fsu)
+  target_link_libraries(hello PRIVATE fsu)
   if(CMAKE_Fortran_PLATFORM_ID MATCHES SunOS)
-    target_link_libraries(hello sunmath m)
+    target_link_libraries(hello PRIVATE sunmath m)
     test_sunquad(CMAKE_HAS_SUNQUAD)
     if(CMAKE_HAS_SUNQUAD)
-      target_link_libraries(hello sunquad)
+      target_link_libraries(hello PRIVATE sunquad)
     endif()
   endif()
 endif()