patch 8.0.1337: typo in #ifdef

Problem:    Typo in #ifdef.
Solution:   Fix the #if line.
diff --git a/src/mbyte.c b/src/mbyte.c
index 54186d9c..8bef7b8 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -6487,7 +6487,7 @@
     void
 im_set_active(int active)
 {
-#  ifdef(USE_IM_CONTROL) && defined(FEAT_EVAL)
+#  if defined(USE_IM_CONTROL) && defined(FEAT_EVAL)
     if (p_imaf[0] != NUL)
 	call_imactivatefunc(p_imdisable ? FALSE : active);
 #  endif