Need -pthread for make on ubuntu 20.04

Makefile 

```makefile
main: ggml.o main.o
	g++ -pthread -o main ggml.o main.o
	./main -h

ggml.o: ggml.c ggml.h
	gcc -pthread -O3 -mavx -mavx2 -mfma -mf16c -c ggml.c

main.o: main.cpp ggml.h
	g++ -pthread -O3 -std=c++11 -c main.cpp
```