tests/libfixmath_unittests: add missing printf_float dependency

The test fix16_str_unittests uses snprintf("%0.5f") to compare to fix16_to_str.

tests/libfixmath_unittests/Makefile

@@ -15,6 +15,8 @@ ifneq (,$(filter native,$(BOARD)))
   export LINKFLAGS += -lm
 endif
 
+USEMODULE += printf_float
+
 include $(RIOTBASE)/Makefile.include
 
 test: