travis: use existing Makefile target to run checksrc

.travis.yml

@@ -101,7 +101,9 @@ install:
 script:
   - |
       if [ "$B" = "style" ]; then
-        perl src/checksrc.pl -i4 -m79 -ASIZEOFNOPAREN -ASNPRINTF -ACOPYRIGHT -AFOPENMODE -Wsrc/libssh2_config.h src/*.[ch] include/*.h example/*.c
+        ./buildconf
+        ./configure
+        make checksrc
       fi
   - |
       if [ "$B" = "configure" ]; then