Fixed exhaustively upstream in https://github.com/kcat/openal-soft/commit/64f8e8347f7ed3f98917fae7587f4d72b54c28ef
but doesn't apply cleanly, so do a minimal fix here.
--- a/alc/alu.h
+++ b/alc/alu.h
@@ -2,6 +2,7 @@
 #define ALU_H
 
 #include <bitset>
+#include <cstdint>
 
 #include "aloptional.h"
 
--- a/core/mixer/defs.h
+++ b/core/mixer/defs.h
@@ -2,6 +2,7 @@
 #define CORE_MIXER_DEFS_H
 
 #include <array>
+#include <cstdint>
 #include <stdlib.h>
 
 #include "alspan.h"
--- a/core/uhjfilter.h
+++ b/core/uhjfilter.h
@@ -2,6 +2,7 @@
 #define CORE_UHJFILTER_H
 
 #include <array>
+#include <cstdint>
 
 #include "almalloc.h"
 #include "alspan.h"