gdb/testsuite: compile gdb.linespec/cp-completion-aliases.exp as C++

Noticed in passing that the prepare_for_testing call in
gdb.linespec/cp-completion-aliases.exp does not pass the 'c++' flag,
despite this being a C++ test.

I guess, as the source file has the '.cc' extension, all the compilers
are doing the right thing anyway -- the source file uses templates, so
is definitely being compiled as C++.

I noticed this when I tried to set CXX_FOR_TARGET (but not
CC_FOR_TARGET) and spotted that this script was still using the C
compiler.

Fixed in this commit by adding the 'c++' flag for prepare_for_testing.