Add Lily58 serial to LIB_SRC to avoid LTO issue (qmk#7868)

This code is timing sensitive and seems to break with LTO enabled (at
least on avr-gcc 8.3.0... it worked on older gcc versions).

This is the same workaround as qmk#7558 applied for the Helix.

keyboards/lily58/rules.mk

@@ -36,6 +36,11 @@ SRC += i2c.c
 SRC += serial.c
 SRC += ssd1306.c
 
+# A workaround until #7089 is merged.
+# serial.c must not be compiled with the -lto option.
+# The current LIB_SRC has a side effect with the -fno-lto option, so use it.
+LIB_SRC += serial.c
+
 # if firmware size over limit, try this option
 # CFLAGS += -flto