.gitignore: handle subprojects better

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I773e3c8abd743c751bec3a2e9dd689603c7f5cf0
diff --git a/.gitignore b/.gitignore
index de8712e..6da54be 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 build*/
-subprojects/*/
+subprojects/*
+!subprojects/*.wrap