ndk-sys: Fail bindings generation when sysroot is not found

Mistyping the env var path resulted in obscure "header not found" errors
rather than `libclang` complaining that the path passed to `--sysroot`
wasn't valid in the first place.

ndk-sys/generate_bindings.sh

@@ -1,5 +1,8 @@
 #!/bin/sh
 
+sysroot="${ANDROID_NDK_ROOT}"/toolchains/llvm/prebuilt/linux-x86_64/sysroot/
+[ ! -d "$sysroot" ] && echo "Android sysroot $sysroot does not exist!" && exit 1
+
 while read ARCH && read TARGET ; do
     bindgen wrapper.h -o src/ffi_$ARCH.rs \
         --blocklist-item 'JNI\w+' \
@@ -47,9 +50,7 @@ while read ARCH && read TARGET ; do
         --newtype-enum 'cryptoinfo_mode_t' \
         --newtype-enum 'log_id' \
         -- \
-        --sysroot="${ANDROID_NDK_ROOT}"/toolchains/llvm/prebuilt/linux-x86_64/sysroot/ --target=$TARGET
-
-        # --newtype-enum '_bindgen_ty_63' \
+        --sysroot="$sysroot" --target=$TARGET
 done << EOF
 arm
 arm-linux-androideabi