fix: unknown CMake command "check_function_exists" (pytorch#126165)

When building pytorch with OpenBLAS on windows I ran into this CMake issue:

```
CMake Error at cmake/Modules/FindLAPACK.cmake:137 (check_function_exists):
  Unknown CMake command "check_function_exists".
Call Stack (most recent call first):
  cmake/Dependencies.cmake:1745 (find_package)
  CMakeLists.txt:708 (include)
```

Similarly described here: https://discuss.pytorch.org/t/cmake-with-error-by-compiling-on-windows-with-mingw32-make/159140

This PR fixes this issue by adding:

```
include(CheckFunctionExists)
```

To the offending CMake file.
Pull Request resolved: pytorch#126165
Approved by: https://github.com/ezyang

cmake/Modules/FindLAPACK.cmake

@@ -26,6 +26,7 @@ ENDIF(LAPACK_FIND_QUIETLY OR NOT LAPACK_FIND_REQUIRED)
 
 # Old search lapack script
 include(CheckFortranFunctionExists)
+include(CheckFunctionExists)
 
 macro(Check_Lapack_Libraries LIBRARIES _prefix _name _flags _list _blas)
   # This macro checks for the existence of the combination of fortran libraries