build: symtab: Add a dummy entry at the end for the binary search

The `symtab_find_symbol_name()` is using an adapted binary
search function to get the entry between 2 addresses, we need
to add a dummy entry at the end so that the search function
can remain simple and straightforward without doing
out-of-bound checks:

   20  \
       |
       |
   50  x
       |
       |
   90  x
    .  |
    .  |
    .  |
dummy  /

Signed-off-by: Yong Cong Sin <ycsin@meta.com>