gitignore: ignore .exp files, as generated by the MS linker on win32

Ignore another filetype, as generated by Microsofts lib.exe when creating the import libraries.
(cherry picked from commit 7321163011)

.gitignore

@@ -10,6 +10,7 @@
 *.def
 *.dll
 *.lib
+*.exp
 config.*
 doc/*.1
 doc/*.html