remove GCC visibility pragmas

libnotmuch.so.* linking fail on some environments. According to
David Bremner on irc:
 "We jump through hoops with the linker script (notmuch.sym) so
 the pragmas are not needed. And they are a little bizarre in a
 library anyway..."

util/xutil.h

@@ -25,8 +25,6 @@
 #include <sys/types.h>
 #include <regex.h>
 
-#pragma GCC visibility push(hidden)
-
 /* xutil.c */
 void *
 xcalloc (size_t nmemb, size_t size);
@@ -51,6 +49,4 @@ int
 xregexec (const regex_t *preg, const char *string,
 	  size_t nmatch, regmatch_t pmatch[], int eflags);
 
-#pragma GCC visibility pop
-
 #endif