Add POSIX swab.

Surprisingly to me, there are actual uses of `swab` in the codebases
I have available to search, including one with a #ifndef __ANDROID__
around it.

Bug: N/A
Test: ran tests
Change-Id: Ic91b78ae22bb65c346cb46dd38916f48d979abe0
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index eafbbd7..0f1e325 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1391,6 +1391,7 @@
     sethostent;
     setnetent;
     setprotoent;
+    swab;
     syncfs;
 } LIBC_O;