Switch to -fstack-protector-all

The gcc that Travis CI uses does not known
-fstack-protector-strong yet:

$ make
gcc  -Wall -Wextra -DVERSION=\"v1.0-9-g29c1e52\" -g -fstack-protector-strong -o earlyoom kill.c main.c meminfo.c
gcc: error: unrecognized command line option ‘-fstack-protector-strong’
make: *** [earlyoom] Error 1

Makefile

@@ -1,5 +1,5 @@
 VERSION ?= $(shell git describe --tags --dirty 2> /dev/null)
-CFLAGS += -Wall -Wextra -DVERSION=\"$(VERSION)\" -g -fstack-protector-strong
+CFLAGS += -Wall -Wextra -DVERSION=\"$(VERSION)\" -g -fstack-protector-all
 
 DESTDIR ?=
 PREFIX ?= /usr/local