bpo-15913: Implement PyBuffer_SizeFromFormat() (GH-13873)

Implement PyBuffer_SizeFromFormat() function (previously
documented but not implemented): call struct.calcsize().
diff --git a/Include/cpython/abstract.h b/Include/cpython/abstract.h
old mode 100644
new mode 100755
index 62a113f..04e4a9e
--- a/Include/cpython/abstract.h
+++ b/Include/cpython/abstract.h
@@ -243,7 +243,7 @@
 
 /* Return the implied itemsize of the data-format area from a
    struct-style description. */
-PyAPI_FUNC(int) PyBuffer_SizeFromFormat(const char *);
+PyAPI_FUNC(Py_ssize_t) PyBuffer_SizeFromFormat(const char *format);
 
 /* Implementation in memoryobject.c */
 PyAPI_FUNC(int) PyBuffer_ToContiguous(void *buf, Py_buffer *view,