parisc: pci-dma: fix warning unused-function

When building tinyconfig on parisc the following warnign shows up:

/tmp/arch/parisc/kernel/pci-dma.c:338:12: warning: 'proc_pcxl_dma_show' defined but not used [-Wunused-function]
 static int proc_pcxl_dma_show(struct seq_file *m, void *v)
            ^~~~~~~~~~~~~~~~~~

Mark the function as __maybe_unused to fix the warning.
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/kernel/pci-dma.c

@@ -335,7 +335,7 @@ pcxl_free_range(unsigned long vaddr, size_t size)
 	dump_resmap();
 }
 
-static int proc_pcxl_dma_show(struct seq_file *m, void *v)
+static int __maybe_unused proc_pcxl_dma_show(struct seq_file *m, void *v)
 {
 #if 0
 	u_long i = 0;