Fix signedness error in 9f225e992 for gcc

The first argument of vshrq_n_s8 needs to be a signed vector type,
but it was passed unsigned. Clang is more lax with conversion, but
gcc needs a cast.

Fix by me, tested by Masahiko Sawada

Per buildfarm members splitfin, batta, widowbird, snakefly, parula,
massasauga

Discussion: https://postgr.es/m/20240306074106.mg6w4koohdlworbs%40alap3.anarazel.de

src/include/port/simd.h

@@ -323,7 +323,7 @@ vector8_highbit_mask(const Vector8 v)
 		1 << 4, 1 << 5, 1 << 6, 1 << 7,
 	};
 
-	uint8x16_t	masked = vandq_u8(vld1q_u8(mask), (uint8x16_t) vshrq_n_s8(v, 7));
+	uint8x16_t	masked = vandq_u8(vld1q_u8(mask), (uint8x16_t) vshrq_n_s8((int8x16_t) v, 7));
 	uint8x16_t	maskedhi = vextq_u8(masked, masked, 8);
 
 	return (uint32) vaddvq_u16((uint16x8_t) vzip1q_u8(masked, maskedhi));