move __stdio_exit_needed to stdio_impl.h

this functions is glue for linking dependency logic.
diff --git a/src/internal/stdio_impl.h b/src/internal/stdio_impl.h
index 4cb7986..cdb0f96 100644
--- a/src/internal/stdio_impl.h
+++ b/src/internal/stdio_impl.h
@@ -61,6 +61,8 @@
 int __toread(FILE *);
 int __towrite(FILE *);
 
+void __stdio_exit_needed(void);
+
 #if defined(__PIC__) && (100*__GNUC__+__GNUC_MINOR__ >= 303)
 __attribute__((visibility("protected")))
 #endif
diff --git a/src/stdio/__toread.c b/src/stdio/__toread.c
index 35f67b8..8f9f27b 100644
--- a/src/stdio/__toread.c
+++ b/src/stdio/__toread.c
@@ -13,8 +13,6 @@
 	return (f->flags & F_EOF) ? EOF : 0;
 }
 
-void __stdio_exit_needed(void);
-
 void __toread_needs_stdio_exit()
 {
 	__stdio_exit_needed();
diff --git a/src/stdio/__towrite.c b/src/stdio/__towrite.c
index b022cbc..0199867 100644
--- a/src/stdio/__towrite.c
+++ b/src/stdio/__towrite.c
@@ -17,8 +17,6 @@
 	return 0;
 }
 
-void __stdio_exit_needed(void);
-
 void __towrite_needs_stdio_exit()
 {
 	__stdio_exit_needed();