CMakeLists.txt: Fix warning on gcc-9

/root/rpmbuild/BUILD/openssl-gost-engine-1.1.1/CMakeFiles/CMakeTmp/src.c:4:14: warning: initialization of 'int *' from incompatible pointer type 'char *' [-Wincompatible-pointer-types]
    4 |     int *p = buf + 1;
      |              ^~~
/root/rpmbuild/BUILD/openssl-gost-engine-1.1.1/CMakeFiles/CMakeTmp/src.c:5:14: warning: initialization of 'int *' from incompatible pointer type 'char *' [-Wincompatible-pointer-types]
    5 |     int *q = buf + 2;
      |              ^~~

Reported-by: Ilya Shipitsin <https://github.com/chipitsine>
Fixes: #288
(cherry picked from commit 6c7addf78b7fe7c8841d4cda6c9d710e4992c7a6)