unix/modtermios: Provide B57600 and B115200 constants only if defined.

unix/modtermios.c

@@ -136,8 +136,12 @@ STATIC const mp_rom_map_elem_t mp_module_termios_globals_table[] = {
     C(TCSANOW),
 
     C(B9600),
+    #ifdef B57600
     C(B57600),
+    #endif
+    #ifdef B115200
     C(B115200),
+    #endif
 #undef C
 };