fix(sgx-dcap-quoteverify-sys): successfull build with SGX_SDK unset

If `SGX_SDK` is unset, a single `-I` argument is passed to the C
compiler, which leads to an unsuccesful build, even if the needed header
files are present in the standard include path.

Signed-off-by: Harald Hoyer <harald@matterlabs.dev>

Author: haraldh

QuoteVerification/dcap_quoteverify/sgx-dcap-quoteverify-sys/build.rs

@@ -42,22 +42,25 @@ fn main() {
     // Tell cargo to invalidate the built crate whenever the wrapper changes
     println!("cargo:rerun-if-changed=bindings.h");
 
+
+    // The bindgen::Builder is the main entry point
+    // to bindgen, and lets you build up options for
+    // the resulting bindings.
+    let mut builder = bindgen::Builder::default();
+
     // Set sdk to search path if SGX_SDK is in environment variable
-    let mut sdk_inc = String::from("-I");
     if let Ok(val) = env::var("SGX_SDK") {
+        let mut sdk_inc = String::from("-I");
         sdk_inc.push_str(&val);
         sdk_inc.push_str("/include/");
+        // Include search path
+        builder = builder.clang_arg(sdk_inc);
     }
 
-    // The bindgen::Builder is the main entry point
-    // to bindgen, and lets you build up options for
-    // the resulting bindings.
-    let bindings = bindgen::Builder::default()
+    let bindings = builder
         // The input header we would like to generate
         // bindings for.
         .header("bindings.h")
-        // Include search path
-        .clang_arg(sdk_inc)
         // Convert C enum to Rust enum
         .rustified_enum("_quote3_error_t")
         .rustified_enum("_sgx_ql_request_policy")