Debugger: Fix compile for g++ 4.7.2

src/plugins/debugger/gdb/gdbengine.cpp: In lambda function:
src/plugins/debugger/gdb/gdbengine.cpp:3224:30: error: the value of 'this' is not usable in a constant expression
src/plugins/debugger/gdb/gdbengine.cpp:3210:61: note: 'this' was not declared 'constexpr'
src/plugins/debugger/gdb/gdbengine.cpp:3231:13: error: 'incmd' was not declared in this scope
This reverts one code block of commit
2b16b97f

Change-Id: Iab7f5ec84e59d9f280e0405cdd82705c14bb2064
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>