Turn on -Wmissing-declarations following MalcolmRobb#67 cleaning up a few places that triggered it

Author: mutability

Makefile

@@ -6,7 +6,7 @@ BLADERF ?= yes
 CPPFLAGS += -DMODES_DUMP1090_VERSION=\"$(DUMP1090_VERSION)\" -DMODES_DUMP1090_VARIANT=\"dump1090-fa\"
 
 DIALECT = -std=c11
-CFLAGS += $(DIALECT) -O2 -g -Wall -Werror -W -D_DEFAULT_SOURCE -fno-common
+CFLAGS += $(DIALECT) -O2 -g -Wall -Wmissing-declarations -Werror -W -D_DEFAULT_SOURCE -fno-common
 LIBS = -lpthread -lm -lrt
 
 ifeq ($(RTLSDR), yes)