[ruby/etc] Declare `getlogin` only if unistd.h is not available

https://github.com/ruby/etc/commit/365398ea47

ext/etc/etc.c

@@ -14,6 +14,8 @@
 #include <sys/types.h>
 #ifdef HAVE_UNISTD_H
 #include <unistd.h>
+#else
+char *getlogin(void);
 #endif
 
 #ifdef HAVE_GETPWENT
@@ -54,7 +56,6 @@ static VALUE sGroup;
 #  include <stdlib.h>
 # endif
 #endif
-char *getlogin();
 
 #define RUBY_ETC_VERSION "1.4.2"
 
@@ -68,7 +69,8 @@ void rb_deprecate_constant(VALUE mod, const char *name);
 
 #ifndef HAVE_RB_IO_DESCRIPTOR
 static int
-io_descriptor_fallback(VALUE io) {
+io_descriptor_fallback(VALUE io)
+{
     rb_io_t *fptr;
     GetOpenFile(io, fptr);
     return fptr->fd;