Ruby: Fix compilation errors on gcc 4.7 and clang 3.2
Ruby: Fix compilation errors on gcc 4.7 and clang 3.2

This patch fixes a few (recently added) errors that prevented gem5 from
compiling on more recent versions of gcc and clang.

LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,haowu4682/gem5,LingxiaoJIA/gem5,haowu4682/gem5,haowu4682/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,LingxiaoJIA/gem5,haowu4682/gem5