perf utils: Be consistent about minimum text size in the svghelper

Be more consistent in the svghelper about the minimum text size
by having a global #define for this.

There needs to be a minimum text size in order to keep the size
of the SVG file within the reach of what current SVG viewers can
cope with.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Cc: fweisbec@gmail.com
Cc: peterz@infradead.org
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arjan van de Ven <arjan@infradead.org>
LKML-Reference: <20090919133507.7374ef8b@infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>