[SPEC2017] Add CMAKE_C_FLAGS `-Wno-implicit-int` for 527.cam4_r. (#284)

Without this patch, an error will occurs:
```
cpu2017/benchspec/CPU/527.cam4_r/src/mpi.c:23:1: error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]
   23 | FORT_NAME( mpi_init_fort , MPI_INIT_FORT)
      | ^
      | int
```