[libc][Obvious] Fix incorrect filepath for ftell.h header

Summary:
The previous patch moved the location of this CMake line but didn't
update the header. Fix it.

Author: jhuber6

libc/src/stdio/generic/CMakeLists.txt

@@ -101,7 +101,7 @@ add_entrypoint_object(
   SRCS
     ftell.cpp
   HDRS
-    ftell.h
+    ../ftell.h
   DEPENDS
     libc.src.errno.errno
     libc.include.stdio