gitignore: minor fixup for symlink workflow

It is convenient to have the subprojects source populated by symlinking
to a common workspace so that cross-repository development can be done.
The previous .gitignore caused 'untracked file' for each subproject
symlink because it was a symlink instead of a directory.  Modify the
.gitignore to match what most other meson repositories have done.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I303effd3c8e7099aaade4a3ac1e81ac7f55cdec0
diff --git a/.gitignore b/.gitignore
index 9a5b17a..8b0c636 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,4 +9,5 @@
 
 # Meson
 /build*/
-/subprojects/*/
+/subprojects/*
+!/subprojects/*.wrap