stm32/Makefile: Include copysign.c in double precision float builds.

This is required for DEBUG=1 builds when MICROPY_FLOAT_IMPL=double.

Thanks to Andrew Leech.

ports/stm32/Makefile

@@ -136,6 +136,7 @@ SRC_LIBM = $(addprefix lib/libm_dbl/,\
 	ceil.c \
 	cos.c \
 	cosh.c \
+	copysign.c \
 	erf.c \
 	exp.c \
 	expm1.c \