[gdb/testsuite] Fix valgrind-infcall-2.exp without libc debug info

[gdb/testsuite] Fix valgrind-infcall-2.exp without libc debug info

When running test-case gdb.base/valgrind-infcall-2.exp on a system without
libc debug info installed, I run into:
...
(gdb) p printf ("bla")^M
'printf' has unknown return type; cast the call to its declared return type^M
(gdb) FAIL: gdb.base/valgrind-infcall-2.exp: do printf
...

Fix this by casting the result of the printf call to int.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-07-20  Tom de Vries  <tdevries@suse.de>

	* gdb.base/valgrind-infcall-2.exp: Handle printf unknown return type.