Fix incorrect submodule error message in Makefile

This commit fixes a condition where the incorrect submodule error
message was displayed as the check for the directory returns true only
if the directory does not exist

Signed-off-by: Harry Ramsey <harry.ramsey@arm.com>
diff --git a/Makefile b/Makefile
index ebe8492..2e4df67 100644
--- a/Makefile
+++ b/Makefile
@@ -6,7 +6,7 @@
     ifeq (,$(wildcard framework/exported.make))
         # Use the define keyword to get a multi-line message.
         # GNU make appends ".  Stop.", so tweak the ending of our message accordingly.
-        ifeq (,$(wildcard .git))
+        ifneq (,$(wildcard .git))
             define error_message
 ${MBEDTLS_PATH}/framework/exported.make not found (and does appear to be a git checkout). Run `git submodule update --init` from the source tree to fetch the submodule contents.
 This is a fatal error