gitignore: explicitly set unignored subprojects

Additional wrap files will be added during builds by suprojects like
PDI, etc. Thus, we'd better explicitly set the wraps we want to keep.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I18fa6d975f5215495141656e41f5608974b236c8
diff --git a/.gitignore b/.gitignore
index 6da54be..4524188 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,6 @@
 build*/
 subprojects/*
-!subprojects/*.wrap
+!subprojects/phosphor-dbus-interfaces.wrap
+!subprojects/phosphor-logging.wrap
+!subprojects/sdbusplus.wrap
+!subprojects/sdeventplus.wrap