Fix objdump output parser in "nsis.py"

Fix objdump output parser in "nsis.py"

In msys2 distribution objdump from gcc is using single tab character
prefix, but objdump from clang is using 4 white space characters instead.
The script will not identify any dll dependencies for a QEMU build
generated with clang. This in turn will fail the build, because there
will be no files inside dlldir and no setup file will be created.
Instead of checking for whitespace in prefix use lstrip to accommodate
for differences in outputs.

Signed-off-by: Arthur Sengileyev <arthur.sengileyev@gmail.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>