build: Fix *.wrap exclude pattern in .gitignore

The current rules trigger git to to complain with

 $ git add subprojects/openssl.wrap
 The following paths are ignored by one of your .gitignore files:
 subprojects

We need to keep the parent directory in the tracking domain and just
filter out all sub directories and files except the wrap files.

Signed-off-by: Daniel Wagner <[email protected]>

Author: dwsuse

.gitignore

@@ -29,5 +29,5 @@ cscope.*
 
 .build
 
-subprojects
+subprojects/*
 !subprojects/*.wrap