update gitignore file

The libssh library by default does not allow in-source build (with cmake
MacroEnsureOutOfSourceBuild macro). The INSTALL file (implicitly) suggests
creating a build directory. So lets add build to list of git ignore files to
avoid complaints from git.

Signed-off-by: Luka Perkov <luka.perkov@sartura.hr>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

.gitignore

@@ -6,3 +6,4 @@
 obj
 cscope.*
 tags
+build