Let breakpad build with -Wall on OS X and Linux.

A=thakis@chromium.org
Original Review: https://codereview.chromium.org/1550933002/

R=thakis@chromium.org

Review URL: https://codereview.chromium.org/1554613002 .

src/common/linux/synth_elf.h

@@ -173,7 +173,9 @@ class SymbolTable : public Section {
                  uint64_t size, unsigned info, uint16_t shndx);
 
  private:
+#ifndef NDEBUG
   size_t addr_size_;
+#endif
   StringTable& table_;
 };