Fix build with absolute --builddir= values.

[bavshin-f5]

This fixes two issues with auto/configure --builddir=/tmp/nginx-otel-build:

[ 20%] Building CXX object CMakeFiles/ngx_otel_module.dir/src/http_module.cpp.o
In file included from /home/bavshin/sources/nginx/nginx-otel/src/http_module.cpp:1:
/home/bavshin/sources/nginx/nginx-otel/src/ngx.hpp:4:10: fatal error: ngx_config.h: No such file or directory
    4 | #include <ngx_config.h>
      |          ^~~~~~~~~~~~~~

[ 90%] Linking CXX shared module /home/bavshin/sources/nginx/nginx/tmp/nginx-otel-build/ngx_otel_module.so