scripts: elf_helper: Tidy up get_symbols() to eliminate pylint warning

Using a member variable in the dict comprehension was probably a typo
(can't see 'sym' referenced elsewhere). Use a local variable instead.

Made pylint spit out these warnings (which might be spurious though):

    scripts/elf_helper.py:535:24: E0203: Access to member 'sym' before
    its definition line 536 (access-member-before-definition)

    scripts/elf_helper.py:535:39: E0203: Access to member 'sym' before
    its definition line 536 (access-member-before-definition)

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>