test_librbd.cc: fix sizeof() in malloc call

test_librbd.cc: fix sizeof() in malloc call

Use 'char' instead of 'char *'.

228  names = (char *) malloc(sizeof(char *) * 1024);
     Result of 'malloc' is converted to a pointer of type 'char',
     which is incompatible with sizeof operand type 'char *'

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>