mm/filemap.c: include <linux/ramfs.h> for generic_file_vm_ops definition

The generic_file_vm_ops is defined in <linux/ramfs.h> so include it to
fix the following warning:

  mm/filemap.c:2717:35: warning: symbol 'generic_file_vm_ops' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/20191008102311.25432-1-ben.dooks@codethink.co.uk
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -40,6 +40,7 @@
 #include <linux/rmap.h>
 #include <linux/delayacct.h>
 #include <linux/psi.h>
+#include <linux/ramfs.h>
 #include "internal.h"
 
 #define CREATE_TRACE_POINTS