*: don't include standard C headers inside of extern "C"

Including C headers inside of `extern "C"` breaks use from C++. Hoist
the includes of standard C headers above the block so we don't try
to mangle the stdlib.

I initially tried to scope this with a targeted change but it's too
hard to do correctly that way. This way, we avoid whack-a-mole.

Firefox is working around this in their e21461b7b8b39cc31ba53c47d4f6f310c673ff2f
commit.

Bug: https://bugzilla.mozilla.org/1953080

spa/include/spa/buffer/alloc.h

@@ -5,12 +5,12 @@
 #ifndef SPA_BUFFER_ALLOC_H
 #define SPA_BUFFER_ALLOC_H
 
+#include <spa/buffer/buffer.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include <spa/buffer/buffer.h>
-
 #ifndef SPA_API_BUFFER_ALLOC
  #ifdef SPA_API_IMPL
   #define SPA_API_BUFFER_ALLOC SPA_API_IMPL

spa/include/spa/buffer/buffer.h

@@ -5,13 +5,13 @@
 #ifndef SPA_BUFFER_H
 #define SPA_BUFFER_H
 
+#include <spa/utils/defs.h>
+#include <spa/buffer/meta.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include <spa/utils/defs.h>
-#include <spa/buffer/meta.h>
-
 #ifndef SPA_API_BUFFER
  #ifdef SPA_API_IMPL
   #define SPA_API_BUFFER SPA_API_IMPL

spa/include/spa/buffer/meta.h

@@ -5,13 +5,13 @@
 #ifndef SPA_META_H
 #define SPA_META_H
 
+#include <spa/utils/defs.h>
+#include <spa/pod/pod.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include <spa/utils/defs.h>
-#include <spa/pod/pod.h>
-
 #ifndef SPA_API_META
  #ifdef SPA_API_IMPL
   #define SPA_API_META SPA_API_IMPL

spa/include/spa/buffer/type-info.h

@@ -5,6 +5,10 @@
 #ifndef SPA_BUFFER_TYPES_H
 #define SPA_BUFFER_TYPES_H
 
+#include <spa/buffer/buffer.h>
+#include <spa/buffer/meta.h>
+#include <spa/utils/type.h>
+
 /**
  * \addtogroup spa_buffer
  * \{
@@ -14,10 +18,6 @@
 extern "C" {
 #endif
 
-#include <spa/buffer/buffer.h>
-#include <spa/buffer/meta.h>
-#include <spa/utils/type.h>
-
 #define SPA_TYPE_INFO_Buffer			SPA_TYPE_INFO_POINTER_BASE "Buffer"
 #define SPA_TYPE_INFO_BUFFER_BASE		SPA_TYPE_INFO_Buffer ":"