fix: Add space between string literals and PRId64 macros

Fixes C++11 literal-suffix warnings in util.h, net.h, and alert.cpp.
Required space between string literal and macro per C++11 standard.

No functional changes - formatting only.

src/alert.cpp

@@ -53,8 +53,8 @@ std::string CUnsignedAlert::ToString() const
     return strprintf(
         "CAlert(\n"
         "    nVersion     = %d\n"
-        "    nRelayUntil  = %"PRId64"\n"
-        "    nExpiration  = %"PRId64"\n"
+        "    nRelayUntil  = %" PRId64 "\n"
+        "    nExpiration  = %" PRId64 "\n"
         "    nID          = %d\n"
         "    nCancel      = %d\n"
         "    setCancel    = %s\n"

src/net.h

@@ -422,7 +422,7 @@ public:
         // the key is the earliest time the request can be sent
         int64_t& nRequestTime = mapAlreadyAskedFor[inv];
         if (fDebugNet)
-            printf("askfor %s   %"PRId64" (%s)\n", inv.ToString().c_str(), nRequestTime, DateTimeStrFormat("%H:%M:%S", nRequestTime/1000000).c_str());
+            printf("askfor %s   %" PRId64 " (%s)\n", inv.ToString().c_str(), nRequestTime, DateTimeStrFormat("%H:%M:%S", nRequestTime/1000000).c_str());
 
         // Make sure not to reuse time indexes to keep things in the same order
         int64_t nNow = (GetTime() - 1) * 1000000;

src/util.h

@@ -237,7 +237,7 @@ void runCommand(std::string strCommand);
 
 inline std::string i64tostr(int64_t n)
 {
-    return strprintf("%"PRId64, n);
+    return strprintf("%" PRId64, n);
 }
 
 inline std::string itostr(int n)