folly: update krb5 include

Summary:
As indicated at https://github.com/krb5/krb5/blob/master/src/include/krb5.h,
this header is deprecated, so update to use the new location (and fix lint).

Reviewed By: luciang

Differential Revision: D9259770

fbshipit-source-id: 6832206b8c890184f5d3369b0305048ae9c377d9

folly/portability/Krb5.h

@@ -24,7 +24,7 @@
 
 #include <gssapi/gssapi_generic.h>
 #include <gssapi/gssapi_krb5.h>
-#include <krb5.h> // @manual
+#include <krb5/krb5.h>
 
 // Kerberos defines a bunch of things that we implement as actual
 // functions, so undefine whatever mess kerberos has done.