Add build folder to .gitignore.

A library is usually expected to be built in a folder /build at
its root. Whenever this repository is added as a submodule of
another project and compiled, git shows all the generated files as
changes. To suppress those git messages, this ignores the build
folder.

.gitignore

@@ -22,6 +22,7 @@
 /minigzipsh
 /zlib.pc
 /configure.log
+/build
 
 .DS_Store
 .vs