pkgconf: pkg-config.in: double quote $@

pkgconf: pkg-config.in: double quote $@

The shell expands "$@" as "$1" "$2" "$3"... while it expands $@ as $1 $2
$3. With the second form, we loses spaces in positional parameters.

As example, the following call

        pkg-config --cflags "one two" three

is wrapped as

        pkgconf --cflags one two three

while we are expecting

        pkgconf --cflags "one two" three

"$@" is really useful when writing wrappers. It passes the positional
arguments *as* they are given.

Double quote $@ to prevent from splitting elements.

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>