Add missing FFI_HIDDEN to ffi_tramp_is_present declaration (#651)

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
diff --git a/include/ffi_common.h b/include/ffi_common.h
index 20aa756..2bd31b0 100644
--- a/include/ffi_common.h
+++ b/include/ffi_common.h
@@ -126,7 +126,7 @@
 
 /* The arch code calls this to determine if a given closure has a
    static trampoline. */
-int ffi_tramp_is_present (void *closure);
+int ffi_tramp_is_present (void *closure) FFI_HIDDEN;
 
 /* Extended cif, used in callback from assembly routine */
 typedef struct