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

src/util/tests/string_buffer/string_buffer_test.cpp

@@ -40,9 +40,9 @@ class string_buffer : public ::testing::Test {
 public:
 
    struct _mesa_string_buffer *buf;
-   const char *str1 = "test1";
-   const char *str2 = "test2";
-   const char *str3 = "test1test2";
+   const char *str1;
+   const char *str2;
+   const char *str3;
    char str4[80];
    char str5[40];
 
@@ -53,6 +53,9 @@ public:
 void
 string_buffer::SetUp()
 {
+   str1 = "test1";
+   str2 = "test2";
+   str3 = "test1test2";
    buf = _mesa_string_buffer_create(NULL, INITIAL_BUF_SIZE);
 }