libc/stdc_has_single_bit.c: fix gcc warning (-Wparentheses)

gcc14 is concerned that the operator precedence between - and & might
be confusing.  Throw in some redundant parentheses to make it shut up.
The LLVM build was fine before this change.

Reported by:	Martin Filla <freebsd@sysctl.cz>
Approved by:	markj (mentor)
MFC after:	1 month
Fixes:		6296500a85
Differential Revision:	https://reviews.freebsd.org/D54057

lib/libc/stdbit/stdc_has_single_bit.c

@@ -10,29 +10,29 @@
 bool
 stdc_has_single_bit_uc(unsigned char x)
 {
-	return (x != 0 && (x & x - 1) == 0);
+	return (x != 0 && (x & (x - 1)) == 0);
 }
 
 bool
 stdc_has_single_bit_us(unsigned short x)
 {
-	return (x != 0 && (x & x - 1) == 0);
+	return (x != 0 && (x & (x - 1)) == 0);
 }
 
 bool
 stdc_has_single_bit_ui(unsigned int x)
 {
-	return (x != 0 && (x & x - 1) == 0);
+	return (x != 0 && (x & (x - 1)) == 0);
 }
 
 bool
 stdc_has_single_bit_ul(unsigned long x)
 {
-	return (x != 0 && (x & x - 1) == 0);
+	return (x != 0 && (x & (x - 1)) == 0);
 }
 
 bool
 stdc_has_single_bit_ull(unsigned long long x)
 {
-	return (x != 0 && (x & x - 1) == 0);
+	return (x != 0 && (x & (x - 1)) == 0);
 }