[libc] Move getenv back to stdlib

Why was I trying to put getenv in stdio? It belongs in stdlib.

src/libraries/libc/stdio.h

@@ -116,10 +116,6 @@ int feof(FILE *stream);
 int ferror(FILE *stream);
 void perror(const char *s);
 
-// Environment functions
-//
-char * getenv(const char *name);
-
 #ifdef __cplusplus
 } // extern "C"
 #endif

src/libraries/libc/stdlib/getenv.cpp

@@ -1,3 +1,3 @@
-#include <stdio.h>
+#include <stdlib.h>
 
 char * getenv(const char *name) { return nullptr; }