appveyor: pin cbindgen to 0.14.1

0.14.2 breaks builds with Rust 1.34.0, which we still support.

Also build cbdingen in debug mode. It builds much faster
with minimal runtime performance.

appveyor.yml

@@ -58,7 +58,7 @@ install:
 
 build_script:
   - set Path=%MINGW_DIR%\bin;c:\msys64\usr\bin;%PCAP_PATH%;%Path%
-  - cargo install --force cbindgen
+  - cargo install --force --debug --version 0.14.1 cbindgen
   - set Path=C:\Users\appveyor\.cargo\bin;%Path%
   - git clone https://github.com/OISF/libhtp -b 0.5.x
   - bash autogen.sh