[gdb/testsuite] Fix implicit declaration of printf in gdb.objc/*.m
When running gdb.objc/objcdecode.exp we get:
...
objcdecode.m: In function '-[Decode multipleDef]':
objcdecode.m:14:3: warning: incompatible implicit declaration of built-in \
function 'printf'
printf("method multipleDef\n");
^~~~~~
objcdecode.m:14:3: note: include '<stdio.h>' or provide a declaration of \
'printf'
...
Fix this in the three gdb.objc/*.m test-cases by including stdio.h.