gtest.h is missing from the repo

[karel-brinda]

$ g++ -g -Wall -Wno-unused-function -std=c++17 -O2 unittest.cpp -o kmerstest
In file included from unittest.cpp:1:
./parser_unittest.h:8:10: fatal error: 'gtest/gtest.h' file not found
#include "gtest/gtest.h"
         ^~~~~~~~~~~~~~~
1 error generated.
make: *** [kmers] Error 1