tools: PCI: Fix compilation warnings

tools: PCI: Fix compilation warnings

Current compilation produces the following warnings:

tools/pci/pcitest.c: In function 'run_test':
tools/pci/pcitest.c:56:9: warning: unused variable 'time'
[-Wunused-variable]
  double time;
         ^~~~
tools/pci/pcitest.c:55:25: warning: unused variable 'end'
[-Wunused-variable]
  struct timespec start, end;
                         ^~~
tools/pci/pcitest.c:55:18: warning: unused variable 'start'
[-Wunused-variable]
  struct timespec start, end;
                  ^~~~~
tools/pci/pcitest.c:146:1: warning: control reaches end of non-void
function [-Wreturn-type]
 }
 ^

Fix them:
 - remove unused variables
 - change function return from int to void, since it's not used

Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
[lorenzo.pieralisi@arm.com: rewrote the commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Kishon Vijay Abraham I <kishon@ti.com>