dt_mipmap_cache_init(): fix build with clang

i386, clang-3.8

[ 16%] Building C object src/CMakeFiles/lib_darktable.dir/common/mipmap_cache.c.o
/home/lebedevri/darktable/src/common/mipmap_cache.c:464:67: error: implicit conversion from 'uint64_t' (aka 'unsigned long long') to 'size_t' (aka 'unsigned int') changes value from 8589934592 to 0 [-Werror,-Wconstant-conversion]
  size_t max_mem = CLAMPS(cache_memory, 100u << 20, ((uint64_t)8) << 30);
         ~~~~~~~                                    ~~~~~~~~~~~~~~^~~~~
/home/lebedevri/darktable/src/common/darktable.h:136:58: note: expanded from macro 'CLAMPS'
#define CLAMPS(A, L, H) ((A) > (L) ? ((A) < (H) ? (A) : (H)) : (L))

src/common/mipmap_cache.c

@@ -461,7 +461,7 @@ void dt_mipmap_cache_init(dt_mipmap_cache_t *cache)
   // we want at least 100MB, and consider 8G just still reasonable.
   int64_t cache_memory = dt_conf_get_int64("cache_memory");
   int worker_threads = dt_conf_get_int("worker_threads");
-  size_t max_mem = CLAMPS(cache_memory, 100u << 20, ((uint64_t)8) << 30);
+  size_t max_mem = CLAMPS(cache_memory, 100u << 20, ((size_t)8) << 30);
   const uint32_t parallel = CLAMP(worker_threads, 1, 8);
 
   // Fixed sizes for the thumbnail mip levels, selected for coverage of most screen sizes