[gdb/build, c++20] Fix invalid conversion in test_symbols

[gdb/build, c++20] Fix invalid conversion in test_symbols

When building gdb with -std=c++20, I run into:
...
gdb/dwarf2/read.c:2709:3: error: invalid conversion from ‘const char8_t*’ to \
  ‘const char*’ [-fpermissive]
 2709 |   u8"u8função",
      |   ^~~~~~~~~~~~
      |   |
      |   const char8_t*
...

Fix this by making the conversion explicit.

Tested on x86_64-linux.