libc/stdio: Build the lib_noflush() and lib_snoflush() stubs even if...

libc/stdio:  Build the lib_noflush() and lib_snoflush() stubs even if CONFIG_FILE_DESCRIPTORS=0.  They may still be needed.

configs/z80sim/src/Makefile

@@ -36,6 +36,10 @@
 -include $(TOPDIR)/Make.defs
 
 ASRCS =
-CSRCS = z80_irq.c z80_serial.c z80_timerisr.c z80_lowputc.c
+CSRCS = z80_irq.c z80_timerisr.c z80_lowputc.c
+
+ifneq ($(CONFIG_DISABLE_SIGNALS),y)
+CSRCS += z80_serial.c
+endif
 
 include $(TOPDIR)/configs/Board.mk