Mark a function in assembler-x64-inl.h inline.

Without this annotation, it can be multiply-defined when it is
included in multiple source files.  This is a commit for
evanm@chromium.org.  See http://codereview.chromium.org/2044004


git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4619 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/x64/assembler-x64-inl.h

@@ -34,7 +34,7 @@
 namespace v8 {
 namespace internal {
 
-Condition NegateCondition(Condition cc) {
+inline Condition NegateCondition(Condition cc) {
   return static_cast<Condition>(cc ^ 1);
 }