pkg.m4: add missing quotes in PKG_WITH_MODULES

[MisterDA]

A small reproducer:

AC_INIT([hello], [1.0])
m4_include([pkg.m4])

AC_PROG_CC
PKG_PROG_PKG_CONFIG

PKG_WITH_MODULES([mylib], [libmylib],
  [AC_CHECK_FUNC([printf])],
  [AC_CHECK_FUNC([printf])],
  [message],
  [yes])
AC_OUTPUT

before this patch (your error may vary):

$ autoreconf && ./configure
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether the compiler supports GNU C... yes
checking whether gcc accepts -g... yes
checking for gcc option to enable C11 features... none needed
checking for pkg-config... /usr/bin/pkg-config
checking pkg-config is at least version 0.9.0... yes
./configure: line 3345: syntax error near unexpected token `in'
./configure: line 3345: `        in case <limits.h> declares $2.'

after this patch:

$ autoreconf && ./configure
checking for gcc... gcc
checking whether the C compiler works... yes
checking for C compiler default output file name... a.out
checking for suffix of executables...
checking whether we are cross compiling... no
checking for suffix of object files... o
checking whether the compiler supports GNU C... yes
checking whether gcc accepts -g... yes
checking for gcc option to enable C11 features... none needed
checking for pkg-config... /usr/bin/pkg-config
checking pkg-config is at least version 0.9.0... yes
checking for libmylib... no
checking for printf... yes
configure: creating ./config.status