This shouldn't be needed after 1.2.13 as a big rework landed, see
https://github.com/alsa-project/alsa-lib/commit/ea8972c83b020d92e1a9f0a5c12eaee159bf6c63.

Bug: https://bugs.gentoo.org/946562
Bug: https://bugs.gentoo.org/946626
--- a/include/pcm.h
+++ b/include/pcm.h
@@ -33,6 +33,7 @@
 extern "C" {
 #endif
 
+#include <stddef.h>
 #include <stdint.h>
 
 /**
--- a/include/rawmidi.h
+++ b/include/rawmidi.h
@@ -28,6 +28,8 @@
 #ifndef __ALSA_RAWMIDI_H
 #define __ALSA_RAWMIDI_H
 
+#include <stddef.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
--- a/include/seq.h
+++ b/include/seq.h
@@ -29,6 +29,7 @@
 #ifndef __ALSA_SEQ_H
 #define __ALSA_SEQ_H
 
+#include <stddef.h>
 #include "ump.h"
 
 #ifdef __cplusplus
--- a/include/seq_midi_event.h
+++ b/include/seq_midi_event.h
@@ -28,6 +28,8 @@
 #ifndef __ALSA_SEQ_MIDI_EVENT_H
 #define __ALSA_SEQ_MIDI_EVENT_H
 
+#include <stddef.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
--- a/include/seqmid.h
+++ b/include/seqmid.h
@@ -28,6 +28,8 @@
 #ifndef __ALSA_SEQMID_H
 #define __ALSA_SEQMID_H
 
+#include <stddef.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
--- a/include/timer.h
+++ b/include/timer.h
@@ -28,6 +28,9 @@
 #ifndef __ALSA_TIMER_H
 #define __ALSA_TIMER_H
 
+#include <stddef.h>
+#include <sys/types.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
--- a/include/ump.h
+++ b/include/ump.h
@@ -9,6 +9,9 @@
 #ifndef __ALSA_UMP_H
 #define __ALSA_UMP_H
 
+#include <stddef.h>
+#include <sys/types.h>
+
 #include "rawmidi.h"
 
 #ifdef __cplusplus
--- a/include/ump_msg.h
+++ b/include/ump_msg.h
@@ -9,6 +9,7 @@
 #ifndef __ALSA_UMP_MSG_H
 #define __ALSA_UMP_MSG_H
 
+#include <stddef.h>
 #include <stdint.h>
 
 #ifdef __cplusplus