CMake: Disable vixl compiling if not enabled

Like how FEXCore disabled vixl linking, make sure we aren't accidentally
compiling and including it when vixl stuff is disabled. Noticed this in
the build logs.

CMakeLists.txt

@@ -273,8 +273,10 @@ if (BUILD_TESTS)
   set(COMPILE_VIXL_DISASSEMBLER TRUE)
 endif()
 
-add_subdirectory(External/vixl/)
-include_directories(SYSTEM External/vixl/src/)
+if (COMPILE_VIXL_DISASSEMBLER OR ENABLE_VIXL_SIMULATOR)
+  add_subdirectory(External/vixl/)
+  include_directories(SYSTEM External/vixl/src/)
+endif()
 
 if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   # This means we were attempted to get compiled with GCC