scripts: kernel-doc: fix array element capture in pointer-to-func parsing

Currently, kernel-doc causes an unexpected error when array element (i.e.,
"type (*foo[bar])(args)") is present as pointer parameter in
pointer-to-function parsing.

For e.g., running kernel-doc -none on kernel/gcov/gcc_4_7.c causes this
error:
"Use of uninitialized value $param in regexp compilation at ...", in
combination with:
"warning: Function parameter or member '' not described in 'gcov_info'"

Here, the parameter parsing does not take into account the presence of
array element (i.e. square brackets) in $param.

Provide a simple fix by adding square brackets in the regex, responsible
for capturing $param.

A quick evaluation, by running 'kernel-doc -none' on entire kernel-tree,
reveals that no additional warning or error has been added or removed by
the fix.

Suggested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Aditya Srivastava <yashsri421@gmail.com>
Tested-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20210217145625.14006-1-yashsri421@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>