tools: PCI: Add a missing option help line

Add a missing option help line for performing legacy interrupt test.

Signed-off-by: Stan Drozd <drozdziak1@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

tools/pci/pcitest.c

@@ -173,6 +173,7 @@ usage:
 			"\t-D <dev>		PCI endpoint test device {default: /dev/pci-endpoint-test.0}\n"
 			"\t-b <bar num>		BAR test (bar number between 0..5)\n"
 			"\t-m <msi num>		MSI test (msi number between 1..32)\n"
+			"\t-l			Legacy IRQ test\n"
 			"\t-r			Read buffer test\n"
 			"\t-w			Write buffer test\n"
 			"\t-c			Copy buffer test\n"