build: exclude dlmalloc.[ch] from checkstyle verification
build: exclude dlmalloc.[ch] from checkstyle verification

- dlmalloc.[ch] is an imported open source library which
  somehow manages to choke clang-format-diff to the point
  of consuming ~1 minute to run against a single character
  diff.

Type: make

Signed-off-by: Dave Wallace <0ed937c0e4b2f09f7bc24acf16cee52b8a199d5f@gmail.com>
Change-Id: I16c6d6da664da8634aa682dce9d2120072626730

FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp