mark get_shell_name FAST_FUNC

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/include/libbb.h b/include/libbb.h
index f22e58e..2059567 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -1248,7 +1248,7 @@
  * Note that getpwuid result might need xstrdup'ing
  * if there is a possibility of intervening getpwxxx() calls.
  */
-const char *get_shell_name(void);
+const char *get_shell_name(void) FAST_FUNC;
 
 #if ENABLE_SELINUX
 extern void renew_current_security_context(void) FAST_FUNC;