boringssl.sh: try suppressing warning in AWS-LC on Windows

```
/home/runner/work/curl-for-win/curl-for-win/awslc/crypto/rand_extra/windows.c:68:22: error: cast from 'FARPROC' (aka 'long long (*)()') to 'ProcessPrngFunction' (aka 'int (*)(unsigned char *, unsigned long long)') converts to incompatible function type [-Werror,-Wcast-function-type-mismatch]
   68 |   g_processprng_fn = (ProcessPrngFunction)GetProcAddress(hmod, "ProcessPrng");
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
```
https://github.com/curl/curl-for-win/actions/runs/11711041033/job/32618698694#step:3:4180

Author: vszakats

boringssl.sh

@@ -66,6 +66,7 @@ _VER="$1"
   [ "${CW_DEV_INCREMENTAL:-}" != '1' ] && rm -r -f "${_PKGDIR:?}" "${_BLDDIR:?}"
 
   CFLAGS="-ffile-prefix-map=$(pwd)="
+  CPPFLAGS=''
   LIBS='-lpthread'  # for tests
   options=''
 
@@ -107,6 +108,8 @@ _VER="$1"
       options+=' -DDISABLE_GO=ON'
       options+=' -DDISABLE_PERL=ON'
 
+      CPPFLAGS+=' -Wno-cast-function-type-mismatch'
+
       # Patch out to avoid redefinition errors
       sed -i.bak 's/-D_WIN32_WINNT=_WIN32_WINNT_WIN7//g' ./CMakeLists.txt
     else
@@ -127,8 +130,8 @@ _VER="$1"
     # shellcheck disable=SC2086
     cmake -B "${_BLDDIR}" ${_CMAKE_GLOBAL} ${_CMAKE_CXX_GLOBAL} ${options} \
       '-DBUILD_SHARED_LIBS=OFF' \
-      "-DCMAKE_C_FLAGS=${_CFLAGS_GLOBAL_CMAKE} ${_CFLAGS_GLOBAL} ${_CPPFLAGS_GLOBAL} ${CFLAGS} ${_LDFLAGS_GLOBAL} ${LIBS}" \
-      "-DCMAKE_CXX_FLAGS=${_CFLAGS_GLOBAL_CMAKE} ${_CFLAGS_GLOBAL} ${_CPPFLAGS_GLOBAL} ${CFLAGS} ${_LDFLAGS_GLOBAL} ${LIBS} ${_CXXFLAGS_GLOBAL} ${_LDFLAGS_CXX_GLOBAL}"
+      "-DCMAKE_C_FLAGS=${_CFLAGS_GLOBAL_CMAKE} ${_CFLAGS_GLOBAL} ${_CPPFLAGS_GLOBAL} ${CFLAGS} ${CPPFLAGS} ${_LDFLAGS_GLOBAL} ${LIBS}" \
+      "-DCMAKE_CXX_FLAGS=${_CFLAGS_GLOBAL_CMAKE} ${_CFLAGS_GLOBAL} ${_CPPFLAGS_GLOBAL} ${CFLAGS} ${CPPFLAGS} ${_LDFLAGS_GLOBAL} ${LIBS} ${_CXXFLAGS_GLOBAL} ${_LDFLAGS_CXX_GLOBAL}"
   fi
 
   cmake --build "${_BLDDIR}"  # --verbose