Missing include for intptr_t

...at least when building with clang-cl against recent MSVC 2019 standard
library:

> external/clew/source/include\clew/clew.h(319,9): error: unknown type name 'intptr_t'
> typedef intptr_t            cl_context_properties;
>         ^

Change-Id: Ib438b680964028461ac3e566410fc90259d015b2
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/122888
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

external/clew/source/include/clew/clew.h

@@ -84,6 +84,8 @@ extern "C" {
 #define CL_EXTENSION_WEAK_LINK
 #endif
 
+#include <stdint.h>
+
 #if defined(_WIN32) && defined(_MSC_VER)
 
 /* scalar types  */
@@ -165,8 +167,6 @@ typedef double          cl_double16[16];
 
 #else
 
-#include <stdint.h>
-
 /* scalar types  */
 typedef int8_t          cl_char;
 typedef uint8_t         cl_uchar;