POSIX: Move C/C++ separation

src/platforms/px4_posix.h

@@ -87,6 +87,8 @@ typedef struct pollfd px4_pollfd_struct_t;
 #define  PX4_F_CREAT  O_CREAT
 #define	 PX4_STACK_OVERHEAD	8192
 
+__BEGIN_DECLS
+
 typedef short pollevent_t;
 
 typedef struct {
@@ -100,8 +102,6 @@ typedef struct {
 	void   *priv;     	/* For use by drivers */
 } px4_pollfd_struct_t;
 
-__BEGIN_DECLS
-
 __EXPORT int 		px4_open(const char *path, int flags, ...);
 __EXPORT int 		px4_close(int fd);
 __EXPORT ssize_t	px4_read(int fd, void *buffer, size_t buflen);