headers: make more headers self-contained

Some IDEs parse headers in isolation to populate their code model. This
exposes some missing headers, mainly for alsa internal struct
definitions, ssize_t (requiring sys/types.h), size_t (requiring
string.h), va_args (requiring stdargs.h) or FILE (requiring stdio.h)

This patch makes the main headers parsable by qtcreator

include/input.h

@@ -29,6 +29,7 @@
 #define __ALSA_INPUT_H
 
 #include <stdarg.h>
+#include <stdio.h>
 
 #ifdef __cplusplus
 extern "C" {