[build] CMake: fix system library names for MinGW (#2219)

.lib extension is used in MSVC and MSVC-like toolchain. In MinGW environment,
.dll.a extension is used for system import libraries.

CMakeLists.txt

@@ -906,7 +906,7 @@ if (srt_libspec_shared)
 			set_target_properties(${TARGET_srt}_shared PROPERTIES LINK_FLAGS "/DELAYLOAD:libeay32.dll")
 		endif()
 	elseif (MINGW)
-		target_link_libraries(${TARGET_srt}_shared PRIVATE wsock32.lib ws2_32.lib)
+		target_link_libraries(${TARGET_srt}_shared PRIVATE wsock32 ws2_32)
 	elseif (APPLE)
 		set_property(TARGET ${TARGET_srt}_shared PROPERTY MACOSX_RPATH ON)
 	endif()