trace-cmd,Makefile: Use ':=' to only execute the find once

When searching for a docbook.xsl file, use ':=' instead of '?=' because
the later executes the shell command every time the variable is referenced.

Introduce a FIND_MANPAGE_DOCBOOK_XSL to use :=, and still keep the
MANPAGE_DOCBOOK_XSL as '?=' to be overwritten. It just uses the FIND version
as a reference instead.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
1 file changed