Add .gitignore entries for AIX-specific intermediate build artifacts.

.gitignore

@@ -9,6 +9,7 @@
 *.sl.[0-9].[0-9]
 *.dylib
 *.dll
+*.exp
 *.a
 *.mo
 *.pot

src/backend/.gitignore

 /postgres
 /postgres.def
+/postgres.imp