ccanlint: fix compile on x86-64

ccanlint: fix compile on x86-64

cc -g -Wall -Wstrict-prototypes -Wold-style-definition -Wmissing-prototypes -Wmissing-declarations -I. -MD -Werror   -c -o tools/ccanlint/tests/examples_run.o tools/ccanlint/tests/examples_run.c
cc1: warnings being treated as errors
tools/ccanlint/tests/examples_run.c: In function ‘scan_forv’:
tools/ccanlint/tests/examples_run.c:37: warning: passing argument 2 of ‘__builtin_va_copy’ discards qualifiers from pointer target type
tools/ccanlint/tests/examples_run.c:43: warning: passing argument 4 of ‘scan_forv’ from incompatible pointer type
tools/ccanlint/tests/examples_run.c:52: warning: passing argument 4 of ‘scan_forv’ from incompatible pointer type
tools/ccanlint/tests/examples_run.c:60: warning: passing argument 4 of ‘scan_forv’ from incompatible pointer type
tools/ccanlint/tests/examples_run.c: In function ‘scan_for’:
tools/ccanlint/tests/examples_run.c:78: warning: passing argument 4 of ‘scan_forv’ from incompatible pointer type
make: *** [tools/ccanlint/tests/examples_run.o] Error 1

It really doesn't like constifying a va_arg, so remove the const declaration.