Avoid conflict with includes from simdutf installed system-wide

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -138,6 +138,7 @@ include_directories(
         "${CMAKE_SOURCE_DIR}/contrib/lua-lpeg"
         "${CMAKE_SOURCE_DIR}/contrib/frozen/include"
         "${CMAKE_SOURCE_DIR}/contrib/fu2/include"
+        "${CMAKE_SOURCE_DIR}/contrib/simdutf/include"
         "${CMAKE_BINARY_DIR}/src" # Stored in the binary dir
         "${CMAKE_BINARY_DIR}/src/libcryptobox"
 )
