:g/\w\+()/exec "norm! J"

:g => Global for multiple repeats
\w\+() => matches pattern, e.g fun_name()
exec  => execute string as command 
norm  => normal mode
norm! => [!] - mapping will not be used 
J     => CTRL-j, move next line to the end of current line 

The interesting point here is "exec" which CHAINs the normal mode: J

In plain English:
Search pattern "\w\+()" globally AND move next line to the end of current line.

Test it:

fun()
{
}

run command => 

fun(){
}