Add __fseterr.

Trivial, obvious counterpart to the standard ferror(3) and clearerr(3),
and lets us build bison out of the box.

Bug: http://b/64273806
Test: ran tests
Change-Id: I20affabddb71210051165c41e86adfe5ae04f77f
diff --git a/libc/libc.x86.map b/libc/libc.x86.map
index 4498d6d..6e9acc0 100644
--- a/libc/libc.x86.map
+++ b/libc/libc.x86.map
@@ -1304,6 +1304,7 @@
   global:
     __freading;
     __free_hook;
+    __fseterr;
     __fwriting;
     __malloc_hook;
     __memalign_hook;