valgrind: Rework test for unsupported platforms

Change things so that on an unsupported platform we will #error rather
than undef the feature.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/include/valgrind/valgrind.h b/include/valgrind/valgrind.h
index e59a7fd..5d4fa5f 100644
--- a/include/valgrind/valgrind.h
+++ b/include/valgrind/valgrind.h
@@ -121,7 +121,9 @@
 #else
 /* If we're not compiling for our target platform, don't generate
    any inline asms.  */
-#  undef CONFIG_VALGRIND
+#  if IS_ENABLED(CONFIG_VALGRIND)
+#    error "Unsupported platform for valgrind"
+#  endif
 #endif