kbuild: fix typo SRCARCH in find_sources

otherwise get the two copy file list in SRCARCH

for cscope:
C symbol: start_kernel

  File           Function            Line
0 proto.h        <global>              11 extern void start_kernel(void );
1 start_kernel.h <global>              10 extern asmlinkage void __init start_kernel(void );
2 head32.c       i386_start_kernel     37 start_kernel();
3 head32.c       i386_start_kernel     37 start_kernel();
4 head64.c       x86_64_start_kernel   85 start_kernel();
5 head64.c       x86_64_start_kernel   85 start_kernel();
6 head_32.S      options              199 cmpb $0,%cl #the first CPU calls start_kernel
7 head_32.S      options              199 cmpb $0,%cl #the first CPU calls start_kernel
8 enlighten.c    xen_start_kernel    1145 start_kernel();
9 enlighten.c    xen_start_kernel    1145 start_kernel();
a lguest.c       lguest_init         1095 start_kernel();
b main.c         start_kernel         513 asmlinkage void __init start_kernel(void )

after the patch:
C symbol: start_kernel

  File           Function            Line
0 proto.h        <global>              11 extern void start_kernel(void );
1 start_kernel.h <global>              10 extern asmlinkage void __init start_kernel(void );
2 head32.c       i386_start_kernel     37 start_kernel();
3 head64.c       x86_64_start_kernel   85 start_kernel();
4 head_32.S      options              199 cmpb $0,%cl #the first CPU calls start_kernel
5 enlighten.c    xen_start_kernel    1145 start_kernel();
6 lguest.c       lguest_init         1095 start_kernel();
7 main.c         start_kernel         513 asmlinkage void __init start_kernel(void )

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>