util: fix in-class initialization of static member

util: fix in-class initialization of static member

Fix a compile error with G++ 4.4

string_buffer_test.cpp:43: error: ISO C++ forbids initialization of
member ‘str1’
string_buffer_test.cpp:43: error: making ‘str1’ static
string_buffer_test.cpp:43: error: invalid in-class initialization of
static data member of non-integral type ‘const char*’

Tested-by: Vinson Lee <vlee at freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103002