Fix -Wsign-compare warnings from gcc

This commit fixes -Wsign-compare warnings from gcc. No functional changes.

It fixes:
```
g++ -c -pipe -std=c++0x -g -Wall -W -D_REENTRANT -DQT_GUI_LIB -DQT_CORE_LIB -DQT_SHARED -I/usr/share/qt4/mkspecs/linux-g++ -I. -I/usr/include/qt4/QtCore -I/usr/include/qt4/QtGui -I/usr/include/qt4 -I. -I../lib -I../externals/tinyxml -Itemp -Itemp -o temp/checkother.o ../lib/checkother.cpp
../lib/checkother.cpp: In function ‘bool astGetSizeSign(const Settings*, const Token*, int*, char*)’:
../lib/checkother.cpp:84:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         if (sz < 8 * settings->sizeof_int)
                ^
```