runqslower: Fix Makefile

Fix undefined reference linker errors when building runqslower with
gcc 7.4.0 on Ubuntu 18.04.
The issue is with misplaced -lelf, -lz options in Makefile:
$(Q)$(CC) $(CFLAGS) -lelf -lz $^ -o $@

-lelf, -lz options should follow the list of target dependencies:
$(Q)$(CC) $(CFLAGS) $^ -lelf -lz -o $@
or after substitution
cc -g -Wall runqslower.o libbpf.a -lelf -lz -o runqslower

The current order of gcc params causes failure in libelf symbols resolution,
e.g. undefined reference to `elf_memory'

Fixes: 9c01546d ("tools/bpf: Add runqslower tool to tools/bpf")
Signed-off-by: Julia Kartseva <hex@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/908498f794661c44dca54da9e09dc0c382df6fcb.1580425879.git.hex@fb.com

tools/bpf/runqslower/Makefile

@@ -41,7 +41,7 @@ clean:
 
 $(OUTPUT)/runqslower: $(OUTPUT)/runqslower.o $(BPFOBJ)
 	$(call msg,BINARY,$@)
-	$(Q)$(CC) $(CFLAGS) -lelf -lz $^ -o $@
+	$(Q)$(CC) $(CFLAGS) $^ -lelf -lz -o $@
 
 $(OUTPUT)/runqslower.o: runqslower.h $(OUTPUT)/runqslower.skel.h	      \
 			$(OUTPUT)/runqslower.bpf.o