Check for disallowed include strings

    error[cxxbridge]: #include relative to `.` or `..` is not supported in Cargo builds
       ┌─ src/main.rs:10:18
       │
    10 │         include!("../header.h");
       │                  ^^^^^^^^^^^^^ #include relative to `.` or `..` is not supported in Cargo builds
       │
       = note: use a path starting with the crate name
diff --git a/syntax/error.rs b/syntax/error.rs
index a60b7da..9597089 100644
--- a/syntax/error.rs
+++ b/syntax/error.rs
@@ -19,6 +19,7 @@
     CXX_STRING_BY_VALUE,
     CXX_TYPE_BY_VALUE,
     DISCRIMINANT_OVERFLOW,
+    DOT_INCLUDE,
     DOUBLE_UNDERSCORE,
     RUST_TYPE_BY_VALUE,
     USE_NOT_ALLOWED,
@@ -54,6 +55,12 @@
     note: Some("note: explicitly set `= 0` if that is desired outcome"),
 };
 
+pub static DOT_INCLUDE: Error = Error {
+    msg: "#include relative to `.` or `..` is not supported in Cargo builds",
+    label: Some("#include relative to `.` or `..` is not supported in Cargo builds"),
+    note: Some("note: use a path starting with the crate name"),
+};
+
 pub static DOUBLE_UNDERSCORE: Error = Error {
     msg: "identifiers containing double underscore are reserved in C++",
     label: Some("reserved identifier"),