CMakeLists: Use correct name for Tremor library.

Fixes #702.

(cherry picked from commit 51b73d0)

Author: icculus

CMakeLists.txt

@@ -472,7 +472,7 @@ if(SDL2MIXER_VORBIS_TREMOR)
         message(STATUS "Using system tremor")
         find_package(tremor REQUIRED)
         if(NOT SDL2MIXER_VORBIS_TREMOR_SHARED)
-            list(APPEND PC_REQUIRES tremor)
+            list(APPEND PC_REQUIRES vorbisidec)
         endif()
     endif()
     if(SDL2MIXER_VORBIS_TREMOR_SHARED)