btrace: fix test build error in gdb.btrace/instruction_history.c

btrace: fix test build error in gdb.btrace/instruction_history.c

On systems with a newer version of GCC the gdb.btrace/instruction_history.exp
test fails to build like this:

    Running .../gdb.btrace/instruction_history.exp ...
    gdb compile failed, .../gdb.btrace/instruction_history.c:
    In function 'main': .../gdb.btrace/instruction_history.c:24:3: warning:
    implicit declaration of function 'loop' [-Wimplicit-function-declaration]
       loop ();
       ^

Declare loop to fix it.

testsuite/
	* gdb.btrace/instruction_history.c (loop): Add declaration.