only declare vfork when needed, else no-mmu build fails

libc/stdio/popen.c

@@ -29,7 +29,6 @@ libc_hidden_proto(dup2)
 libc_hidden_proto(fdopen)
 libc_hidden_proto(pipe)
 libc_hidden_proto(vfork)
-libc_hidden_proto(fork)
 libc_hidden_proto(fclose)
 
 /* uClinux-2.0 has vfork, but Linux 2.0 doesn't */
@@ -38,6 +37,7 @@ libc_hidden_proto(fclose)
 # define vfork fork	
 # define VFORK_LOCK		((void) 0)
 # define VFORK_UNLOCK	((void) 0)
+libc_hidden_proto(fork)
 #endif
 
 #ifdef __UCLIBC_HAS_THREADS__

libc/stdlib/system.c

@@ -15,12 +15,12 @@ libc_hidden_proto(wait4)
 libc_hidden_proto(execl)
 libc_hidden_proto(signal)
 libc_hidden_proto(vfork)
-libc_hidden_proto(fork)
 
 /* uClinux-2.0 has vfork, but Linux 2.0 doesn't */
 #include <sys/syscall.h>
 #if ! defined __NR_vfork
 #define vfork fork	
+libc_hidden_proto(fork)
 #endif
 
 int __libc_system(char *command)