Update .gitignore (facebookresearch#3492)

Summary:
Adding build folder to gitignore, so that they don't show up in the commit tree while building from source

Pull Request resolved: facebookresearch#3492

Reviewed By: junjieqi

Differential Revision: D58171359

Pulled By: asadoughi

fbshipit-source-id: b0efed348769328a3bdbcc13098dcb84cadb6c4f

.gitignore

@@ -7,6 +7,7 @@
 *.dylib
 *.pyc
 *~
+/build/
 /config.*
 /aclocal.m4
 /autom4te.cache/