vte: support reproducibility when debug build is enabled

vte: support reproducibility when debug build is enabled

When debug build is enabled, there is a build path issue in source file
which caused build is not reproducible
...subprojects/simdutf/simdutf.h...
 1 /* auto-generated on 2025-03-17 16:13:41 -0400. Do not edit! */
 2 /* begin file include/simdutf.h */
 3 // /build-dir/vte-0.82.1/subprojects/simdutf/include/simdutf.h:1
 4 #ifndef SIMDUTF_H
...subprojects/simdutf/simdutf.h...

The source file is auto generated by python script amalgamate.py,
apply a fix to the script to use relative path to instead

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>