remove useless "extern" keywords in headers

include/errno.h

@@ -10,7 +10,7 @@ extern "C" {
 #ifdef __GNUC__
 __attribute__((const))
 #endif
-extern int *__errno_location(void);
+int *__errno_location(void);
 #define errno (*__errno_location())
 
 #ifdef __cplusplus

include/resolv.h

@@ -122,7 +122,7 @@ struct res_sym {
 #define RES_PRF_REPLY	0x00002000
 #define RES_PRF_INIT	0x00004000
 
-extern struct __res_state *__res_state(void);
+struct __res_state *__res_state(void);
 #define _res (*__res_state())
 
 struct rrec;

include/sys/file.h

@@ -11,7 +11,7 @@ extern "C" {
 #define LOCK_NB	4
 #define LOCK_UN	8
 
-extern int flock(int, int);
+int flock(int, int);
 
 #endif