cmake : fix passing GGML_PERF compile option

CMakeLists.txt

@@ -47,7 +47,7 @@ else()
     option(WHISPER_SUPPORT_OPENBLAS    "whisper: support for OpenBLAS" OFF)
 endif()
 
-option(WHISPER_PERF                    "whisper: enable perf timings"          OFF)
+option(WHISPER_PERF                    "whisper: enable perf timings" OFF)
 
 # sanitizers
 
@@ -151,6 +151,10 @@ else()
     endif()
 endif()
 
+if (WHISPER_PERF)
+    set(WHISPER_EXTRA_FLAGS ${WHISPER_EXTRA_FLAGS} -DGGML_PERF)
+endif()
+
 #
 # whisper - this is the main library of the project
 #