Added -Wmissing-prototypes

This warning is useful for catching the easy mistake of missing the
keyword static on functions intended to be internal-only.

Missing the static keyword risks symbol polution and misses potential
compiler optimizations.

This is an interesting warning, while useful for libraries such as
littlefs, it's perfectly valid C to not predeclare all functions, and
common in final application binaries.

Relatedly, this warning is re-disabled for the test/bench runner. There
may be a better way to organize the CFLAGS, maybe into separate
LIB/RUNNER CFLAGS, but I'll leave this to future work if our CFLAGS grow
more complicated.

This was motivated by non-static internal-only functions leaking into a
release. Found and fixed by DvdGiessen.

Makefile

@@ -63,6 +63,7 @@ CFLAGS += -fcallgraph-info=su
 CFLAGS += -g3
 CFLAGS += -I.
 CFLAGS += -std=c99 -Wall -Wextra -pedantic
+CFLAGS += -Wmissing-prototypes
 CFLAGS += -ftrack-macro-expansion=0
 ifdef DEBUG
 CFLAGS += -O0
@@ -354,6 +355,7 @@ summary-diff sizes-diff: $(OBJ) $(CI)
 
 ## Build the test-runner
 .PHONY: test-runner build-test
+test-runner build-test: CFLAGS+=-Wno-missing-prototypes
 ifndef NO_COV
 test-runner build-test: CFLAGS+=--coverage
 endif
@@ -405,6 +407,7 @@ testmarks-diff: $(TEST_CSV)
 
 ## Build the bench-runner
 .PHONY: bench-runner build-bench
+bench-runner build-bench: CFLAGS+=-Wno-missing-prototypes
 ifdef YES_COV
 bench-runner build-bench: CFLAGS+=--coverage
 endif