[GDB] Fix compiling error when not enable GDB.

components/gdb/SConscript

@@ -1,20 +1,20 @@
 Import('rtconfig')
 from building import *
 
+src = Glob('*.c')
 comm = 'libcpu/' + rtconfig.ARCH
 if (rtconfig.CPU == 'cortex-m4') or (rtconfig.CPU == 'cortex-m3'):
     comm = 'libcpu/cortexm'
 
 cwd = GetCurrentDir()
 if rtconfig.PLATFORM == 'armcc':
-	src = Glob('*.c') + Glob(comm + '/*.c') + Glob(comm + '/*_rvds.S')
+    src = src + Glob(comm + '/*.c') + Glob(comm + '/*_rvds.S')
 
 if rtconfig.PLATFORM == 'gcc':
-	src = Glob('*.c') + Glob(comm + '/*.c') + Glob(comm + '/*_gcc.S')
+    src = src + Glob(comm + '/*.c') + Glob(comm + '/*_gcc.S')
 
 if rtconfig.PLATFORM == 'iar':
-	src = Glob('*.c') + Glob(comm + '/*.c') + Glob(comm + '/*_iar.S')
-
+    src = src + Glob(comm + '/*.c') + Glob(comm + '/*_iar.S')
 
 CPPPATH = [cwd, cwd + '/' + comm]