*: 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

src/modules/module-client-node/v0/ext-client-node.h

@@ -5,16 +5,16 @@
 #ifndef __PIPEWIRE_EXT_CLIENT_NODE0_H__
 #define __PIPEWIRE_EXT_CLIENT_NODE0_H__
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include <spa/utils/defs.h>
 #include <spa/param/param.h>
 #include <spa/node/node.h>
 
 #include <pipewire/proxy.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define PW_TYPE_INTERFACE_ClientNode            PW_TYPE_INFO_INTERFACE_BASE "ClientNode"
 
 #define PW_VERSION_CLIENT_NODE0		0

src/modules/module-client-node/v0/transport.h

@@ -5,16 +5,16 @@
 #ifndef __PIPEWIRE_CLIENT_NODE0_TRANSPORT_H__
 #define __PIPEWIRE_CLIENT_NODE0_TRANSPORT_H__
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include <string.h>
 
 #include <spa/utils/defs.h>
 
 #include <pipewire/mem.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /** information about the transport region \memberof pw_client_node */
 struct pw_client_node0_transport_info {
 	int memfd;		/**< the memfd of the transport area */