fix "ifdef ENABLE_foo": should always be "#if ENABLE_foo"

function                                             old     new   delta
pack_gzip                                           1729    1789     +60
fill_window                                          220     216      -4
static.gzip_level_config                              24       -     -24
gzip_main                                            275     192     -83
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 1/2 up/down: 60/-111)           Total: -51 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/procps/pstree.c b/procps/pstree.c
index f97e996..bc9f0c9 100644
--- a/procps/pstree.c
+++ b/procps/pstree.c
@@ -34,7 +34,7 @@
 
 struct child;
 
-#ifdef ENABLE_FEATURE_SHOW_THREADS
+#if ENABLE_FEATURE_SHOW_THREADS
 /* For threads, we add {...} around the comm, so we need two extra bytes */
 # define COMM_DISP_LEN (COMM_LEN + 2)
 #else