BUILD: syscalls: remove improper inline statement in front of syscalls

Trying to build with an old gcc and glibc revealed that we must not
state "inline" in our _syscall* definitions since it's already present
in the declaration making use of the _syscall* macros.
diff --git a/include/common/accept4.h b/include/common/accept4.h
index 1130ecc..8981b92 100644
--- a/include/common/accept4.h
+++ b/include/common/accept4.h
@@ -58,7 +58,7 @@
 	return socketcall(SYS_ACCEPT4, args);
 }
 #else
-static _syscall4(int, accept4, int, sockfd, struct sockaddr *, addr, socklen_t *, addrlen, int, flags);
+static inline _syscall4(int, accept4, int, sockfd, struct sockaddr *, addr, socklen_t *, addrlen, int, flags);
 #endif /* VSYSCALL etc... */
 #endif /* USE_MY_ACCEPT4 */
 #endif /* __linux__ && USE_ACCEPT4 */