add missing header to test code

[mkevac]

Fix for this:

make -C tests
make[1]: Entering directory '/home/marko/projects/preeny/preeny/tests'
cc     hello.c   -o hello
hello.c: In function ‘main’:
hello.c:3:2: warning: implicit declaration of function ‘puts’ [-Wimplicit-function-declaration]
  puts("Hello world!");
  ^
make[1]: Leaving directory '/home/marko/projects/preeny/preeny/tests'

> gcc --version
gcc (GCC) 5.1.1 20150618 (Red Hat 5.1.1-4)

[zardus]

Thanks!