Fixes Windows compiler warning about unary minus applied to unsigned int.

src/regexp-macro-assembler-ia32.cc

Index: src/regexp-macro-assembler-ia32.cc
===================================================================
--- src/regexp-macro-assembler-ia32.cc	(revision 1053)
+++ src/regexp-macro-assembler-ia32.cc	(working copy)
@@ -665,7 +665,7 @@
     const int kRegisterZeroEBPOffset = -1;
     __ mov(ecx, kRegisterZeroEBPOffset);
     // Set eax to address of char before start of input.
-    __ lea(eax, Operand(edi, -char_size()));
+    __ lea(eax, Operand(edi, -static_cast<int32_t>(char_size())));
     Label init_loop;
     __ bind(&init_loop);
     __ mov(Operand(ebp, ecx, times_4, +0), eax);