Find libopenblas.dll.a on windows

"lib" is needed for windows.

With this change, you can build whisper.cpp with OpenBLAS's prebuilt DLL.
1. extract a zip from https://github.com/xianyi/OpenBLAS/releases
2. copy the headers in (openblas)/include to the root directory of whisper.cpp
3. invoke cmake with -DCMAKE_LIBRARY_PATH=(openblas)\lib -DWHISPER_SUPPORT_OPENBLAS=ON
4. copy (openblas)/bin/libopenblas.dll to the same directory of whisper.dll after msbuild

ggerganov/whisper.cpp#89 (comment)

CMakeLists.txt

@@ -99,7 +99,9 @@ if (APPLE AND NOT WHISPER_NO_ACCELERATE)
 endif()
 
 if (WHISPER_SUPPORT_OPENBLAS)
-    find_library(OPENBLAS_LIB openblas)
+    find_library(OPENBLAS_LIB
+        NAMES openblas libopenblas
+        )
     if (OPENBLAS_LIB)
         message(STATUS "OpenBLAS found")