tools lib: Adopt skip_spaces() from the kernel sources

Same implementation, will be used to replace ad-hoc equivalent code in
tools/.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: André Goddard Rosa <andre.goddard@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-dig691cg9ripvoiprpidthw7@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/include/linux/string.h

@@ -29,4 +29,6 @@ static inline bool strstarts(const char *str, const char *prefix)
 	return strncmp(str, prefix, strlen(prefix)) == 0;
 }
 
-#endif /* _LINUX_STRING_H_ */
+extern char * __must_check skip_spaces(const char *);
+
+#endif /* _TOOLS_LINUX_STRING_H_ */

tools/lib/string.c

@@ -17,6 +17,7 @@
 #include <string.h>
 #include <errno.h>
 #include <linux/string.h>
+#include <linux/ctype.h>
 #include <linux/compiler.h>
 
 /**
@@ -106,3 +107,16 @@ size_t __weak strlcpy(char *dest, const char *src, size_t size)
 	}
 	return ret;
 }
+
+/**
+ * skip_spaces - Removes leading whitespace from @str.
+ * @str: The string to be stripped.
+ *
+ * Returns a pointer to the first non-whitespace character in @str.
+ */
+char *skip_spaces(const char *str)
+{
+	while (isspace(*str))
+		++str;
+	return (char *)str;
+}