Move String::SizeOf<> to string.h .

string.h was already indirectly (through class.h) including
everything needed for String::SizeOf<>, so move it to the
string.h and make those includes direct. Then we can change
the object-inl.h to include only string.h, reducing the
number of indirectly included files.

Test: m test-art-host-gtest
Test: m testrunner.py --host --optimizing
Change-Id: I51c462c034f205498c539abe6d888be9738d4a6e
diff --git a/runtime/native/java_lang_VMClassLoader.cc b/runtime/native/java_lang_VMClassLoader.cc
index 0630737..b1511c0 100644
--- a/runtime/native/java_lang_VMClassLoader.cc
+++ b/runtime/native/java_lang_VMClassLoader.cc
@@ -20,6 +20,7 @@
 #include "class_linker.h"
 #include "dex/descriptors_names.h"
 #include "dex/dex_file_loader.h"
+#include "dex/utf.h"
 #include "jni/jni_internal.h"
 #include "mirror/class_loader.h"
 #include "mirror/object-inl.h"