[gdb/testsuite] Fix gdb.mi/mi-sym-info.exp on openSUSE Tumbleweed

On openSUSE Tumbleweed, I run into:
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FAIL: gdb.mi/mi-sym-info.exp: List all functions from debug information only
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The problem is in matching this string:
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{name="_start",type="void (void)",description="void _start(void);"}
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using regexp fun_re, which requires a line field:
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set fun_re \
    "\{line=\"$decimal\",name=${qstr},type=${qstr},description=${qstr}\}"
...

Fix this by making the line field optional in fun_re.

Tested on x86_64-linux.