MIPS: MT: Remove "weak" from vpe_run() declaration

Weak header file declarations are error-prone because they make every
definition weak, and the linker chooses one based on link order (see
10629d71

 ("PCI: Remove __weak annotation from pcibios_get_phb_of_node
decl")).

That's not a problem for vpe_run() because Kconfig ensures there's never
more than one definition:

  - vpe_run() is defined in arch/mips/kernel/vpe-mt.c if
    CONFIG_MIPS_VPE_LOADER_MT=y

  - vpe_run() is defined in arch/mips/mti-malta/malta-amon.c if
    CONFIG_MIPS_CMP=y

  - CONFIG_MIPS_VPE_LOADER_MT cannot be set if CONFIG_MIPS_CMP=y

But it's simpler to verify correctness if we remove "weak" from the picture
and test the config symbols directly.

Remove "weak" from the vpe_run() declaration and use #if to test whether a
definition should be present.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: linux-mips@linux-mips.org
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10684/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>