Define UTF8PROC_STATIC macro in the source

This way, there's one less flag you have to pass when building the
library on windows.

lib/build.rs

@@ -35,7 +35,6 @@ fn main() {
     }
 
     config
-        .define("UTF8PROC_STATIC", "")
         .flag_if_supported("-std=c99")
         .flag_if_supported("-Wno-unused-parameter")
         .include("include")

lib/src/lib.c

@@ -1,11 +1,12 @@
 // The Tree-sitter library can be built by compiling this one source file.
 //
 // The following directories must be added to the include path:
-//   - src
 //   - include
-//   - externals/utf8proc
+//   - utf8proc
 
 #define _POSIX_SOURCE
+#define UTF8PROC_STATIC
+
 #include "./get_changed_ranges.c"
 #include "./language.c"
 #include "./lexer.c"