trace-cmd: Check for spaces in character array

Currently when using the raw format for fields, when looking at
a character array, to determine if it is a string or not, we make sure
all characters are "isprint()". If not, then we consider it a numeric
array, and print the hex numbers of the characters instead.

But it seems that '\n' fails the isprint() check! Add isspace() to
the check as well, such that if all characters pass isprint() or
isspace() it will assume the character array is a string.

Reported-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>