fix(bpf): Compile eBPF probe with -Wno-unknown-attributes

In some cases we use clang 7 to build the eBPF probe.
That version of clang is too old to support some of the compiler
attributes which appear in the source code for more modern kernel
versions.  So disable warnings associated with unknown attributes.

Signed-off-by: Joseph Pittman <joseph.pittman@sysdig.com>
Signed-off-by: Luca Guerra <luca@guerra.sh>

driver/bpf/Makefile

@@ -62,5 +62,6 @@ $(obj)/probe.o: $(src)/probe.c \
 		-fno-jump-tables \
 		-fno-stack-protector \
 		-Wno-tautological-compare \
+		-Wno-unknown-attributes \
 		-O2 -g -emit-llvm -c $< -o $(patsubst %.o,%.ll,$@)
 	$(LLC) -march=bpf -filetype=obj -o $@ $(patsubst %.o,%.ll,$@)