Separate multi-words filenames by '_'

Rename foobar.ext to foo_bar.ext.

<https://github.com/Genymobile/scrcpy/pull/226#discussion_r209454865>

app/src/lock_util.h

@@ -0,0 +1,13 @@
+#ifndef LOCKUTIL_H
+#define LOCKUTIL_H
+
+// forward declarations
+typedef struct SDL_mutex SDL_mutex;
+typedef struct SDL_cond SDL_cond;
+
+void mutex_lock(SDL_mutex *mutex);
+void mutex_unlock(SDL_mutex *mutex);
+void cond_wait(SDL_cond *cond, SDL_mutex *mutex);
+void cond_signal(SDL_cond *cond);
+
+#endif