kbuild: install-extmod-build: add missing quotation marks for CC variable

While attempting to build a Debian packages with CC="ccache gcc", I
saw the following error as builddeb builds linux-headers-$KERNELVERSION:

  make HOSTCC=ccache gcc VPATH= srcroot=. -f ./scripts/Makefile.build obj=debian/linux-headers-6.14.0-rc1/usr/src/linux-headers-6.14.0-rc1/scripts
  make[6]: *** No rule to make target 'gcc'.  Stop.

Upon investigation, it seems that one instance of $(CC) variable reference
in ./scripts/package/install-extmod-build was missing quotation marks,
causing the above error.

Add the missing quotation marks around $(CC) to fix build.

Fixes: 5f73e7d0 ("kbuild: refactor cross-compiling linux-headers package")
Co-developed-by: Mingcong Bai <jeffbai@aosc.io>
Signed-off-by: Mingcong Bai <jeffbai@aosc.io>
Tested-by: WangYuli <wangyuli@uniontech.com>
Signed-off-by: WangYuli <wangyuli@uniontech.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>