Don't use VLAs in adb.

This makes no measurable difference to the sync time; "adb sync" of
everything on /system for a Nexus 9 still takes 20s.

Change-Id: Ifa2626f7453937e43856b9c4ee06e1f5db0aa273
diff --git a/adb/Android.mk b/adb/Android.mk
index 2538e2e..c507905 100644
--- a/adb/Android.mk
+++ b/adb/Android.mk
@@ -14,6 +14,7 @@
     -Wall -Wextra -Werror \
     -Wno-unused-parameter \
     -Wno-missing-field-initializers \
+    -Wvla \
     -DADB_REVISION='"$(adb_version)"' \
 
 # Define windows.h and tchar.h Unicode preprocessor symbols so that