Externals: add hidapi

Current master (d17db57b9d4354752e0af42f5f33007a42ef2906) because
Android support was added after the current release (0.8.0-rc1).

Externals/hidapi/linux/.gitignore

@@ -0,0 +1,18 @@
+Debug
+Release
+*.exp
+*.ilk
+*.lib
+*.suo
+*.vcproj.*
+*.ncb
+*.suo
+*.dll
+*.pdb
+*.o
+*.so
+hidtest-hidraw
+.deps
+.libs
+*.lo
+*.la