valgrind: fix ptest compilation for PowerPC64

valgrind: fix ptest compilation for PowerPC64

The fix is similar to what was done for PowerPC32.
It solves below error, while compiling for PowerPC64,

-- snip --
| ../../../../valgrind-3.13.0/none/tests/ppc64/test_isa_2_06_part2.c: In function 'usage':
| ../../../../valgrind-3.13.0/none/tests/ppc64/test_isa_2_06_part2.c:1778:3: warning: implicit declaration of function 'fprintf' [-Wimplicit-function-declaration]
|    fprintf(stderr,
|    ^~~~~~~
| ../../../../valgrind-3.13.0/none/tests/ppc64/test_isa_2_06_part2.c:1778:3: warning: incompatible implicit declaration of built-in function 'fprintf'
| ../../../../valgrind-3.13.0/none/tests/ppc64/test_isa_2_06_part2.c:1778:3: note: include '<stdio.h>' or provide a declaration of 'fprintf'
| ../../../../valgrind-3.13.0/none/tests/ppc64/test_isa_2_06_part2.c:1778:11: error: 'stderr' undeclared (first use in this function)
|    fprintf(stderr,
|            ^~~~~~
-- snip --

Signed-off-by: Jagadeesh Krishnanjanappa <jkrishnanjanappa@mvista.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>