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/timer.h

@@ -28,6 +28,13 @@
 #ifndef __ALSA_TIMER_H
 #define __ALSA_TIMER_H
 
+#include <string.h>
+
+#include <sys/types.h>
+
+#include <alsa/conf.h>
+#include <alsa/global.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif